Live Commentary: Petra Kvitova vs. Flavia Pennetta - as it happened

Good afternoon! It's the second quarter-finals day at Flushing Meadows today, with the remaining two matches in the women's draw up first. Victoria Azarenka will take on Simona Halep later, but first up it's former Wimbledon champion Petra Kvitova and 26th seed Flavia Pennetta.

Pennetta missed the 2012 tournament through injury, but has reached the quarter-finals in her last four appearances at Flushing Meadows. In 2013 she made her only Grand Slam semi-final appearance to date, where she was beaten by Victoria Azarenka, who could be the potential last-four opponent for today's winner.

Despite having reached at least the semi-finals at the other three majors, Kvitova is in the quarters in New York for the first time in her career. She's only reached the fourth round twice before this year. However, for the last five years, she's come into this event after a good run at Wimbledon, but went out in round three it SW19 this year, so may be getting the benefit of an extra week off earlier in the summer.

This will be the seventh time that these two players have come up against each other. Pennetta won their first three meetings, including a Federation Cup semi-final, but Kvitova has won the most recent three. However, this is the first time that they have met since Kvitova knocked the Italian out in the round of 16 at the Olympics in 2012.

Kvitova ended the run of British player Johanna Konta in round four. Konta had only won one Grand Slam match before this tournament, but lost in two closely-fought sets to the Czech, who is the fifth seed at this event. It looks like she spent her day off seeing the sites in New York:

Do I look like a New Yorker or what?! 🗽#petrainnyc pic.twitter.com/lFt0H4dvqs — Petra Kvitova (@Petra_Kvitova) September 8, 2015

Pennetta had a potentially trick match in the fourth round, facing former US Open champion Samantha Stosur. The Australian couldn't reproduce the tennis that handed her the title four years ago though, as the Italian sailed through in straight sets.

The players have just made their way out to the Arthur Ashe Stadium and are going through their warmups ahead of their quarter-final. It's not full in the main stadium yet, but both players receive a warm welcome from the New York crowd.

Right then, the players are ready to go, with Kvitova serving first.

BREAK! Pennetta is on the attack from the start as she slices a forehand winner down the sideline, before Kvitova tries to do the same, only to fire her effort long, bringing up a break point for the Italian. Kvitova comes inside the baseline to attack on the forehand trying to save it, but she makes her third unforced error of the game to hand it to the 26th seed.

BREAK BACK! Kvitova quickly puts the mistakes of the first game behind her, playing to Pennetta's forehand to force a mistake from the Italian, before coming into the net to punch a forehand of her won to the back corner to bring up two break points of her own. Pennetta misses her first serve, giving the Czech a chance to come in again, which she does, swiftly leveling the set.

Kvitova continues to make errors on her own serve, allowing Pennetta to take the game to 30-30. She comes inside the baseline looking to replicate the forehands that won her the previous game, but misses the baseline to bring up another break point for Pennetta, but it's swiftly saved with an ace. The fifth seed finds her forehand to get advantage, but double faults on the next point to keep the Italian in the game. Kvitova finds her focus though, following up a forehand winner with a tidy backhand down the line to hold serve.

Kvitova is on the hunt early on in the game, clearly boosted after saving break points in the previous game. A forehand winner from the Czech is followed by a missed effort out wide from the 26th seed, giving her opponent a 40-15 lead, but more unforced errors from the two-time Grand Slam champion let Pennetta back in. She serves well to force a wayward return from Kvitova, before getting the better in a baseline exchange with the fifth seed to seal the game.

For a third game in a row on Kvitova's serve, Pennetta brings up a break point, hitting a string of steady shots from the baseline, eventually forcing a long forehand from her opponent. However, Kvitova survives as a low backhand from the Italian fails to clear the net. Pennetta finds the net again on the next point, before a long return allows the Czech to hold serve. Kvitova certainly hasn't played her best tennis in her first three service games, but she's doing just enough to stay with her opponent.

BREAK! Kvitova's returning game really has been excellent at the start of this first set, and a stunning forehand from the fifth seed gives her two break points. She wastes the first with a sloppy backhand, but Pennetta hands her the game on the next point with a double fault.

Pennetta is clearly desperate to get the break straight back, pulling off an excellent forehand return down the sideline on her way to a 30-0 lead. Kvitova responds, upping the power on her serve to edge ahead. The Czech gets a time violation warning for taking too long between points, and it seems to affect her focus as she makes two double faults in a row, giving Pennetta a break point, but she regains her composure to get a big serve in, taking the game back to deuce. Kvitova gets a chance to take the game when she brings up advantage with a rare volley, slicing the ball to the back of the court, before Pennetta hits her return long off the next point. Kvitova really lost her head for a couple of points in that game, and it lucky to come out of it without dropping her serve. The scoreline definitely flatters for the former Wimbledon champion at the moment.

Kvitova tries to change the pace up with a couple of sliced shots, but her backhand hits the net chord before dropping back on her side of the court. Pennetta tries to get her opponent moving behind the basline, and it results in a wide effort down the sideline from Kvitova. However, the Czech has the chance to serve for the opening set next.

BREAK! Kvitova looks to be heading towards a one-set lead as she fires her second ace of the match past her opponent. However, she makes back-to-back double faults for the second game in a row to give Pennetta a chance to get the break back. The Italian goes for an ambitious forehand down the sideline, but hits it a fraction late, slicing it wide, before scuffing a backhand into the net on the next point. Kvitova has a set point, but makes her third double fault of the game. Pennetta takes her next couple of returns as early as possible to put her opponent under pressure, and it has the desired effect, with Kvitova missing a backhand before watching a forehand from the Italian fly past her. It's well-played by the 26th seed, but Kvitova really crumbled towards the end of that game.

SET! Kvitova looks completely flat at the start of the game as she makes two more forehand errors, hitting both long, before missing a backhand return to quickly hand Pennetta a 40-0 lead. It looks like the Italian is heading towards a comfortable hold to level the set, but she makes a double fault, before Kvitova hits two remarkable backhand winners to take the game to deuce. It's an impressive turnaround by the Czech. She gets advantage as a powerful return forces a wayward backhand from the 26th seed, and Pennetta makes another double fault to hand her opponent the set. The Italian smashing her racquet in anger as the players head back to their seats.

Pennetta senses a chance to get an early break on the board in the second set after taking a 30-15 lead. She plays a drop shot, which is well executive, but Kvitova shows remarkable pace coming into the net to get it back in play, before making her seventh double fault of the afternoon to take the game to deuce. Kvitova doesn't seem comfortable at all as she calls for her towel between points to dry her hands before slicing a forehand into the net to give Pennetta a chance to break, but a backhand winner keeps her in the game. An ace gives Kvitova advantage though, before Pennetta's return goes long to allow the Czech to escape with her serve intact.

Kvitova fails to get Pennetta's first three serves back in play as the Italian cruises towards a comfortable hold. Kvitova does get the ball back while 40-0 down, but the Italian easily guides it down the sideline for a backhand winner. That might be the most comfortable hold of serve we've seen all match!

Kvitova manages to get through a service game without facing a break point as Pennetta struggles to get her returns back in, before watching a Kvitova forehand fly past her to secure the game for the fifth seed. While Kvitova hasn't looked settled at all, the longer this match goes on, it is likely to play into Kvitova's hands, as she's spent less time on court reaching this stage, and unlike Pennetta hasn't been involved in the doubles.

BREAK! At 30-30, Pennetta has to get down low to lift the ball back across the net, but she hits it long to give Kvitova the first chance to get a break on the board in this set, and she gifts the fifth seed to game when she blasts her forehand well long off the next point.

BREAK BACK! Just as it looks like Kvitova might be starting to take control of the match, the errors creep back in as she sends a forehand long before clattering a volley into the net, while a wayward forehand brings up three break points for Pennetta, which the Italian claims with an excellent backhand.

Kvitova continues to be hampered by unforced errors as she quickly slips 40-0 behind. Pennetta tries something different, coming into the net in a bid to volley, but mis-times her shot. Kvitova can't get her return back off the next point though, allowing Pennetta to level the scores in the second set.

Kvitova serves well at the start of the game, following up an ace with another powerful delivery, but Kvitova hits it straight back with a backhand winner. At deuce, the fifth seed pulls off a stunning cross-court two-handed backhand to bring up advantage, before following it up with another winner on the backhand to seal the game. Given some of the errors she's made during her own service games in the second set, that was a solid effort from the Czech.

Pennetta is starting to look really solid during her service games, following up a cross-court backhand with a forehand winner down the sideline on her way to a 40-15 lead, before Kvitova finds the net with her backhand, keeping the set on serve. The longer the set goes on, the more comfortable the Italian looks, despite having played more tennis at this year's US Open than her opponent.

BREAK! Kvitova is starting to look a little weary, making yet another double fault before carelessly hitting her forehand into the net, giving Pennetta a chance to break. The Italian tries to seal it with a drop shot, but Kvitova sprints in to to it to deuce. The fifth seed misses a backhand though, allowing the Italian to seal with with a stunning backhand down the line. She'll serve the level the match next.

SET! Kvitova battles to keep herself in the game, making a volley close into the net to take the game to 30-30, but Pennetta just clips the baseline with a forehand, despite a challenge from Kvitova. The fifth seed saves it, but the Italian quickly gets another chance as she makes a drop shot, which Kvitova can't reach in time. The Czech pulls off a couple of stunning returns to give herself a chance to break, but the resilient Italian twice takes it back to deuce, before hitting some consistent ground strokes to force more mistakes from her opponent, handing her the set. Both players had moments of brilliance during that game, but it was the 26th seed who kept her nerve.

This match going the distance could cause a bit of a headache for the tournament organisers. There are three matches scheduled for the day session, with Federer and Gasquet coming up in the night session, as well as an exhibition doubles match. However, there's some horrible weather forecast for overnight, so it has just been announced that the Federer match will now be at 7pm local time (midnight in the UK). With another ladies match to go before Wawrinka and Anderson though, that could be cutting it fine. With the weather potentially disrupting tomorrow's play too, It's vital that all the quarter-finals get finished today.

Kvitova is serving first again in the deciding set, and has to be careful as the Italian is quickly into the net to keep the game level. The Czech keep Pennetta at the back of the court on the next point, forcing her to miss a backhand, before another miss from the 26th seed goes long.

It looks like it's going to be an easy hold for the Italian as she opens up a 40-15 lead, but Kvitova fights back with a pair of winners to the back of the court. The Czech misses a backhand while looking for the break point though, and looks rather flat-footed as she makes another error on the backhand to give the Italian the game. Despite the weather that's due overnight, it's still a hot day in New York, and it looks like Kvitova was feeling the heat a bit towards the end of that game.

Kvitova is still taking a while between points, and it almost constantly reaching for her towel, but she's hitting her serves well, with Pennetta getting just two returns in throughout the game, leaving the fifth seed to seal it with an easy forehand after coming inside the baseline. It is worth remembering that Kvitova has been struggling with glandular fever in the run up to the tournament, which could be slowing her movement around the court.

Kvitova has had some success at the net today, despite using her shots there sparingly. She comes in for a volley early in this game though, helping her to a 30-0 lead. Pennetta hits a run of excellent serves though, resulting in more errors from the Czech to allow her to keep the set on serve.

BREAK! Three unforced errors cost Kvitova badly as Pennetta brings up two break points, returning excellently from the back of the court. Kvitova tries to responds, hitting an excellent backhand winner, but she looks exhausted as she gets ready for the next point, which ends in a long forehand from the fifth seed. It's advantage Pennetta in the deciding set.

Pennetta consolidates the break with ease as a drained Kvitova makes mistake after mistake, firing a backhand long before a forehand goes wide. On paper, it should be Pennetta who is struggling with the longer match, having played two more sets than Kvitova to get this far, while also competing in the women's doubles with compatriot Sara Errani, but it's the Czech who is flagging.

BREAK! Kvitova looks like she's going to try to make a fight of it as she comes into the net for another well-taken volley at the start of the game, but follows it up with a double fault. Kvitova comes inside the baseline again, hitting an excellent overhead smash to edge in front. The Italian fights back with a forehand, before back-to-back long shots from the Czech gift the 26th seed the game. Pennetta will serve for the match next.

Flavia Pennetta has done it! The Italian has come from a set down to beat fifth seed Petra Kvitova, progressing to the semi-finals at the US Open for the second time in her career. Kvitova continued to struggle in the final game, making more errors to hand the 26th seed two match points. A wide backhand from Pennetta briefly gave the Czech hope, but Kvitova missed her own backhand on the next point to send her opponent through.

Next up for Pennetta will be either Victoria Azarenka or Simona Halep. They're on court next to decide the final place in the women's semi-finals. Stick around, as we'll be starting our live coverage of that match in just a few minutes.