Third seed Serena Williams took on fellow American Sloane Stephens in the last of the women's quarter-finals at Rod Laver Arena.
Both women were strong on their serve in the opening set, but Williams took the vital break to go 1-0 up.
In the second set an injury hampered the five-time champion and Stephens took the final two sets for a remarkable win.
Read below how the action unfolded.
Hello and thanks for joining me for today's second match at the Australian Open. It's the final women's quarter-final to decide who faces defending champion Victoria Azarenka.
As a reminder, Azarenka has just got past Russian Svetlana Kuznetsova. Michael Penkman's report of the game is here.
The coin toss has been made, and it will be Williams to serve after the warm-ups.
It's the master against the pupil here. Stephens has all the attributes to go a long way in tennis, but it's tough to look beyond the five-time champion.
2003, 2005, 2007, 2009, 2010. Those are the years Williams has won this tournament. This stage is brand new territory for Stephens, but she's built in the mould of her opponent.
Just a minute left of practice and we'll be underway.
Here we go then, Williams to serve! I think it'll be straight sets, but Stephens should be able to nick a few games.
A nice, comfortable hold to kick things off from Serena. Stephens gets a go now. If she can hold it'll be a big boost for her.
Superb forehand from the youngster and she's held very comfortably there. Great start.
Serena's serve is just so powerful. If she gets it in it's so hard to return, as Stephens is finding. 30-0.
A hold to love from Williams. No problem at all with her serve so far. In the last match we were still in the first game after seven minutes. Here, we've had three games flash by.
Nothing at all wrong with the Stephens serve either. She's raced into a 40-0 lead.
Serena goes long and another service game flashes by. At this rate we'll be in a tie break before 3am.
A huge serve finishes another comfortable service game. Although Stephens did manage one point from it. She's already done better than most players can hope to against the powerhouse that is Wiliams.
The dominance of each player's serve is leading to a not very competitive contest at the moment. Fascinating, certainly, but there's not been a sniff of a break yet.
I'm not sure how many points have gone against serve so far, but I'm willing to bet it's five or under. It's exhibition stuff at the moment.
In fact, it's just two won against serve; one for each player. Five games have been held to love and the other two to 15. The longest rally has been about eight shots too.
Williams showing some threat now - she latched on to a Stephens drop shot to hit the winner and go up 0-30.
BREAK! Out of nothing, we have a break. Stephens did well to recover and save two break points, but the third was put into the net by Stephens and Williams breaks at a good time. It was a simple upping of the ante and immediate success.
SET! Williams finally meets some resistance on her serve, but she holds safely to take the opener 6-3. Just 28 minutes that took.
Here's a good time for me to take a break. Enjoy the company of Michael Penkman while I'm gone. If the next set is as quick I'll be back just as it ends. Over to you Michael.
Thanks Matt. Well, Serena stepped it up a gear at the important moment of that set. How often does she do that? You can't really see past her now.
BREAK! Stephens makes a few costly errors and faces two break points. Williams rifles into the net on the first one, but the youngster pushes an inside-out forehand just wide and Williams gets an early break.
Williams really beginning to find the lines now. She worked Stephens around the court then before a lovely cross-court backhand won her the point. She's dictating the pace, and you feel that she still has lots left in the tank.
A 118mph serve has Stephens bunting the ball back into Williams's court, and the third seed rushes to the net and dispatches a forehand winner. Serena's playing at half capacity, if that...
Well, the youngster holds to love. She finishes with a superb forehand winner, but will have to produce much more of that if she is to force a deciding set.
Stephens definitely has the ability. She had to do some work on the defensive in that last point, but did so brilliantly. The youngster turned the point around, before coming into the net to put away a volley. Deuce.
BREAK! An unforced error from Williams brings up Stephens's first break point. She came into the net after another ferocious serve had Stephens just getting the ball back into play, but put her forehand beyond the baseline. Serena then went long with another shot and Stephens has the break back.
Another good hold from Stephens and she has now won three games in a row.
Serena hits a forehand winner. On the half volley. From the baseline. Only she can produce a shot like that.
Stephens is putting in a shift on the baseline, chasing down all Williams can throw at her at the moment. That's bringing out a few errors in Serena's game.
Stephens hits a brilliant winner to bring up break point, but Williams answer with an ace.
Serena finds a couple of big serves to hold. Game on at the moment though.
Williams is getting a bit down on herself at the moment. Stephens gave her an easy chance to pass with a poor approach shot into the net, but the forehand was too long.
Stephens holds for the concession of just one point. She's right in this second set.
Stephens is doing to Williams what Williams normally does to others. She's bossing the game at the moment. Quite extraordinary.
Serena lets out a little yelp as she comes to the net. She seems to be moving a bit gingerly, but has a bigger problem... she faces two break points.
BREAK! Williams is definitely struggling. She's limping a bit and muttering to herself. The serve suffered as a result and Stephens is in control!
Right, I'm handing back over to Matt. He'll take you through what's happening at the moment. Fascinating stuff, it could be over...
Thanks Michael. I take back over with Stephens 30-15 up serving for the second set.
It's now 40-30 as Williams doesn't bother chasing a high defensive shot that Stephens volleys into the open court.
BREAK! Write off Serena at your peril. She got a huge piece of help from a Stephens double fault and then forces the youngster into going long to take the break. Stephens lost that then and Serena has called the trainer.
This was just building up to go into a perhaps surprising third set, but we're again back on serve. Williams is struggling with her back this time, and not her ankle like before.
It will be interesting to see how this affects Williams. The ankle injury from before had no lasting effects and only caused minor discomfort, but a back injury could really hamper her powerful style.
A big service game for her now. It won't take long to see if the injury will give her problems.
Her first serve was very tentative indeed. She's going to have to switch up her game here. She won that first point, though, with some accurate, if not powerful, forehands.
Serena is visibly in pain, but she's 40-0 up.
Williams holds. Her serves were well under the 100mph mark there, but her overall play was more than good enough. Stephens should be doing more, in my opinion.
Williams has still got enough about her to trouble Stephens, but really the youngster is missing too many shots. That said, she recovers from break point down to hold after some good net play.
Stephens is now 0-30 ahead on Williams's next serve. She has a chance here...
SET! We're going to a third set here! After superb defensive tennis from Stephens Williams goes long to set up break point, and the youngster grabs it at the first time of asking - well, Serena throws it away with a forehand into the net. Stephens could well win this.
Stunning standing return from Serena to get herself in this game at 15-30.
Stephens starts the set exactly as she needed to, with a decent service hold. Big game now with Williams serving.
Serena does indeed hold, and that was important. It will give her belief, but the injury has made her a much less attacking player now.
The tennis is not of a particularly high quality at the moment. Stephens holds serve comfortably, but there's something of a lack of urgency. Stephens doesn't seem to want to hammer the nail into the coffin of her compatriot just yet, and has taken the foot off the gas a bit.
A little faster now is Serena's serve, but her accuracy is just failing her at the moment.
Williams hits the net a couple of times in a row and it's break point Stephens...
Boom! A big first serve followed by a smash levels the scores.
A very big hold here for Williams. She fends off that break point to tie up this final set. More importantly, she's starting to find some power with her serve and net shots.
Excellent tennis from Stephens. She runs Williams around the court, right to left, before blocking a strong forehand with a neat volley to win the point. She serves an unreturnable one to lead 3-2 in the set.
The Williams backhand is still working well, and she ties it up at 30-30.
Serena is in agony here, but she's hanging on in there. Gutsy stuff to hold again, surely she can't win this match?
Big baseline rally ends with Stephens looking to wrap it up but Serena is on to the volley and finishes with a forehand.
Stephens goes long and Williams has two break points...
A very minor long call and a forehand into the net sees Williams lose both of them. Deuce.
BREAK! An incredibly close game is just edged by Serena. A fantastic backhand return sets up break point and the game is won with Stephens firing into the net after an impossible return from Williams. That could be the deciding break.
Serena's serve is back. Almost. Not quite as powerful but she's getting stronger now.
Whoops. That said, a double fault gives Stephens two break back points.
BREAK! Well, the youngster just doesn't know when she's beaten. She gets into the second serve and Williams nets a backhand to level it once more.
What a sublime drop shot that is from Stephens at 15-30 down. Soft hands there and Williams was never going to reach it.
Williams responds with some huge baseline shots to level at deuce.
Williams gets a break point but Stephens saves it with a forehand right down the line from wide of the net.
Whatever Stephens can do, Williams can do better! She flies into a forehand with even more power than her opponent to save a game point of her own.
A huge hold for Stephens after saving a couple of break points. She's one away from a place in the semi-finals now, can she break here?
GAME, SET AND MATCH! She can! Incredible from the youngster as she not only holds her nerve but goes at the Williams serve to take the win! Obviously Serena is injured but there was still a lot of work to be done.
Wow, a quite remarkable match. The best of the women's draw so far and it is Sloane Stephens, and not Serena Williams, who will face Victoria Azarenka in the semi-finals.
That's all from me with this one. Join me soon for Andy Murray against Jeremy Chardy.
