Red Bull's Max Verstappen set the pace in the first practice session at the Bahrain Grand Prix in Malaysia.
The Dutchman, who was back on familiar territory after he won the last race at Sepang in 2017, crossed the line 0.383 seconds faster than Mercedes' George Russell.
Isack Hadjar in the other Red Bull finished in third, with Ferrari's Charles Leclerc behind.
Title leader Kimi Antonelli was off the pace by over a second in the heavily-upgraded Mercedes to go fifth-fastest.
The venue is hosting the Bahrain Grand Prix after the race was cancelled in April due to the conflict in the Middle East.
It was a session of experimentation with several of the drivers not even teenagers when F1 last raced in Malaysia nine years ago.
Russell said over the team radio that the track was "very cool," but many of the drivers were hampered by tyre degradation on what was a very dusty track.
Leclerc was running on the medium type at the time when he referred to the degradation as "insane," while Antonelli said the grip was "crazy low."
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Ferrari's Lewis Hamilton finished the session in sixth with Alpine's Pierre Gasly and Racing Bulls' Liam Lawson in seventh and eighth respectively.
Both the McLarens went under the radar with Oscar Piastri down in 11th and Lando Norris one place behind.
The second practice session goes green at 9 a.m. BST.
