Lewis Hamilton set the pace on the final day of pre-season testing in Bahrain to underline his Mercedes team's status as favourites for the season-opener in Australia.
Hamilton was just 0.06 seconds off the fastest time of the winter, set by Williams's Felipe Massa on Saturday.
Valtteri Bottas was second fastest on Sunday, showing Williams, who had a poor 2013 season, could cause an upset.
Ferrari's Fernando Alonso was third while Red Bull's struggles continued.
World champion Sebastian Vettel suffered a spin caused by an apparent brake failure and ended the day eighth fastest, 4.190 seconds slower than Hamilton's best.
But after the car failed to complete even one lap on Saturday, it was a better day for Red Bull in terms of reliability - the world champion completed 77 laps, his highest mileage of the winter on a single day.
"I'm happy we did a lot of laps," Vettel said. "We would love some more laps but testing is over so we have to do more laps down under [when the season starts in Melbourne on 16 March]."
Alonso's fastest lap was the first time a Ferrari had dipped below one minute 35 seconds in Bahrain this winter.
But the Spaniard's lap of one minute 34.280 seconds was still a second off Hamilton's pace in the Mercedes.
Headline lap times are notoriously poor indicators of competitiveness, and that will be even more the case this year following the introduction of a fuel-usage limit into F1.
Nevertheless, Hamilton's pace underlined the already widespread impression his Mercedes team are in the best shape ahead of the new season.
Williams look to be genuine contenders too, following a major restructure of their engineering department over the past few months and their switch from Renault to Mercedes engines.
Like Red Bull, Lotus also continued to struggle, with Romain Grosjean completing only 32 laps before his day was ended by an engine fire.
They have done the least mileage of any team over the winter and, like Red Bull, look a long way from ready for the new season.
Asked about his chances in Australia, Vettel said: "It's very difficult, obviously.
"I am not calling us the favourites - for that we haven't done enough laps and the speed is not there compared to some other teams. So we'll have to wait and see when we get there.
"Things will calm down once we use the two weeks we have until then to fit new parts to the car and build it to the best knowledge that we currently have."