George Russell got his Azerbaijan Grand Prix weekend off to a good start after he topped first practice, while his teammate Kimi Antonelli was forced to end his session early.
The Mercedes driver set a time of 1:45.387, four-tenths quicker than Red Bull's Max Verstappen around the streets of Baku.
Ferrari's Charles Leclerc finished in third, while his teammate Lewis Hamilton sat behind in fourth.
Antonelli ended the session in fifth, eight-tenths off Russell's pace, before he had to stop on the track after reporting a hydraulic failure on his Mercedes.
Near the halfway point of the session, the young Italian, who is leading the world championship by 81 points to Russell, completed only nine laps before suffering the issue.
Carlos Sainz in the heavily-upgraded Williams had a brake fire issue in the pit lane which was put out quickly in the first half of the session.
Other teams which have brought major upgrades this weekend are McLaren and Red Bull, and while Verstappen was looking strong, McLaren spent a lot of the session working on the cars in both sides of the garage.
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Oscar Piastri was reporting on the McLaren team radio that he was experiencing vibrations early on and was sent back to the garage later in the session. The Australian finished sixth overall, over one second off the leader's pace.
His teammate Lando Norris didn't get much running and finished down in 14th.
Racing Bulls' Liam Lawson and Arvid Lindblad finished seventh and eighth.
The second practice session will get going at 1 p.m. UK / 8 a.m. ET.
