Carlos Sainz completed a double race distance for previously struggling Williams and pushed his Ferrari successor Lewis Hamilton off the top of the testing timesheets in Bahrain on Thursday.
Seven times champion Hamilton had been fastest in the morning session but Sainz, doing a full day with a monster 127 laps, lapped the desert Sakhir circuit in one minute 29.348 seconds for the team that finished ninth of 10 last year.
That was 0.031 quicker than Hamilton, who moved from Mercedes at the start of the year, had managed in a morning session interrupted by rain and Sainz was the only driver to do more than 100 laps.
Ferrari chairman John Elkann was on hand to watch proceedings with the Italian team set to start the new campaign with high hopes of winning a championship for the first time since 2008.
Thursday was the second of three days of testing before the season starts in Australia on March 16 and teams were carrying out different programmes with varying fuel levels.
Hamilton's teammate Charles Leclerc was third fastest, 0.083 off Sainz's pace with Mercedes' George Russell second in the morning and ending up fourth overall.
His new 18-year-old Italian teammate Andrea Kimi Antonelli was fifth.
Champions McLaren had Lando Norris, fastest on Wednesday, only 14th but the Briton aborted a lap that was quickest of all in the middle sector.
"We have good innovation on the car. We have not stood still in the winter and I am excited to see it all work. So far so good but it’s obviously early days," team boss Zak Brown told Sky Sports television.
"We have tried to enhance what we had but there is some stuff on there I don’t think has been totally noticed yet, which I think will happen in due course."
Lance Stroll was sixth for Aston Martin ahead of Max Verstappen's new teammate Liam Lawson, who had a full day in the Red Bull and did 91 laps.
Australian rookie Jack Doohan and Alpine teammate Pierre Gasly were eighth and ninth with Racing Bull's Franco-Algerian rookie Isack Hadjar completing the top 10.
McLaren's Oscar Piastri and Sauber's Nico Hulkenberg had a rare testing collision in the morning.
The season starts in Australia on March 16.
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