Pierre Gasly started the Saudi Arabian Grand Prix weekend as the surprise early pacesetter, with the Alpine driver edging Lando Norris and Charles Leclerc in first practice.
The French driver lapped the super-fast Jeddah Corniche circuit with a best time of 1:29.239, 0.007 seconds quicker than McLaren's Formula 1 leader Norris could manage.
Ferrari's Leclerc was third, 0.070 off the leading pace, with McLaren's Australian Oscar Piastri, winner in Bahrain last weekend and only three points behind Norris after four races, fourth.
The session, run in the late afternoon sunshine on the shores of the Red Sea, was largely unrepresentative of the conditions for Saturday's qualifying and Sunday's race, which is held at night.
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But it still served as a useful measure of driver confidence on a daunting track that rewards the gradual building up of pace.
Alex Albon was fifth for Williams with George Russell sixth fastest for Mercedes, ahead of Williams' Carlos Sainz and Ferrari's Lewis Hamilton.
Red Bull's reigning champion Max Verstappen, last year's winner in Jeddah, was ninth and teammate Yuki Tsunoda completed the top 10.
Gasly's Australian rookie teammate Jack Doohan was 16th.
Haas's Oliver Bearman, who made his F1 debut with Ferrari as a stand-in last year, was 18th after a brush with the wall at turn one. He continued without damage.