Lewis Hamilton is predicting the most demanding season of his Mercedes career in his quest for a sixth world championship.
Hamilton has won 51 of the last 100 races en route to securing four titles in five years.
The Silver Arrows are yet to show their true pace at Formula One's opening winter test in Barcelona, with Hamilton only 12th on Wednesday.
Rivals Ferrari, in the hands of Sebastian Vettel and new team-mate Charles Leclerc, have posted a series of headline-grabbing times.
"It is crazy to think I've had six years with the team already, but this is going to be the most challenging year of our partnership," said Hamilton, 34.
"Ferrari are very, very strong right now as you have seen.
"They are racking up great mileage, and it appears that they have a better package than they had last year which means it is going to be even more of a challenge for us this season.
"I have earned the right to be here and I'm still delivering better than ever. I feel in the best shape I have been in for round seven with the team."
On the penultimate day of the opening test at the Circuit de Catalunya, Toro Rosso's Daniil Kvyat was the surprise name at the top of the time charts.
The Russian completed the fastest time of the week with a lap of one minute and 17. 704 seconds to edge out Alfa Romeo driver Kimi Raikkonen.
Trouble-hit Williams finally got their first taste of action after missing the opening two days because their car was not ready.
11 laps on the board for #GR63 so far, as we head into the final hour of Wednesday's running #F1Testing pic.twitter.com/BbyA6oVQqt — ROKiT WILLIAMS RACING (@WilliamsRacing) February 20, 2019
British novice George Russell emerged after the lunch break as Williams bid to play catch-up with just five days of winter testing remaining. He finished last, completing 23 laps.
Deputy team principal Claire Williams said that failure to have the car in place for the opening two and a half days has been humiliating.
"We're not just disappointed," she said. "It's embarrassing not bringing a race car to a circuit when everyone else has managed to do that, particularly a team like ours that has managed to bring a race car to testing for the past 40-odd years."
Speculation surrounds the future of their technical chief Paddy Lowe, and his media commitments were cancelled on Wednesday.
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