Max Verstappen pipped Charles Leclerc to finish fastest in the only practice session of the weekend at the Azerbaijan Grand Prix.
The championship leader set the pace with a time of one-minute 42.315 around the Baku City Circuit, fending off Leclerc by 0.037secs.
Sergio Perez was third with the second Ferrari driver, Carlos Sainz, completing the top four.
Neither Mercedes driver ran the soft tyres, opting to bank an extra fresh set of the compound for use in qualifying.
In a session where teams and drivers desperately wanted to avoid any incident or further limitation to their track time, Mercedes and AlphaTauri endured a dreadful opening few minutes.
In the case of Mercedes, both Lewis Hamilton and George Russell were pulled back into the garage after reporting brake problems.
Yuki Tsunoda then caused his AlphaTauri team a headache by locking his rear tyres and spinning into the wall at turn three. Although the Japanese driver was able to return his car to the pits, he caused significant damage to the right-rear corner after grinding away a partition of the floor and diffuser on the asphalt.
Alpine fire halts play
As inconvenient as these earlier moments were, things were far worse for Alpine after 14 minutes, when Pierre Gasly's A523 caught fire and was forced to stop by the side of the track - something that caused the red flags to fly.
Alpine has confirmed the fire was caused by a hydraulic problem after a loss of pressure was spotted on the telemetry.
With qualifying for Sunday's race the next session scheduled to take place, the ramifications of this failure could be extremely damaging for the team.
At the same time as Gasly stopped, Kevin Magnussen also stopped his Haas on the exit of turn one to retire from the session with a fuel pressure problem.
Aston Martin in trouble
Fernando Alonso has finished third at each of the opening three races of the season but sat in seventh for the majority of FP1 at Baku.
But ahead of the soft tyre qualifying simulations, the Spaniard reported a problem with his DRS. After the team confirmed the system was not working, he pitted for some rapid repairs.
Qualifying simulations
As the clock dipped below the 10-minute point, soft tyres began to be bolted onto the cars with McLaren electing to experiment with Oscar Piastri providing a tow to Lando Norris through the final sector.
At the time, this put him up to fourth, behind only Perez, Verstappen and Sainz.
Azerbaijan Grand Prix practice results
1. Max Verstappen [Red Bull] - 1:42.315
2. Charles Leclerc [Ferrari] - +0.037
3. Sergio Perez [Red Bull] - +0.139
4. Carlos Sainz [Ferrari] - +0.584
5. Lando Norris [McLaren] - +0.810
6. Nyck de Vries [AlphaTauri] - +1.099
7. Lance Stroll [Aston Martin] - +1.140
8. Fernando Alonso [Aston Martin] - +1.245
9. Alex Albon [Williams] - +1.313
10. Zhou Guanyu [Alfa Romeo] - +1.433
11. Lewis Hamilton [Mercedes] - +1.483
12. Oscar Piastri [McLaren] - +1.665
13. Valtteri Bottas [Alfa Romeo] - +1.695
14. Yuki Tsunoda [AlphaTauri] - +1.822
15. Nico Hulkenberg [Haas] - +2.008
16. Logan Sargeant [Williams] - +2.585
17. George Russell [Mercedes] - +2.767
18. Esteban Ocon [Alpine] - +3.640
19. Pierre Gasly [Alpine] - +4.006
20. Kevin Magnussen [Haas] -+5.183
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