Max Verstappen finished fastest during first practice in Barcelona, after George Russell complained of "bouncing" in his Mercedes in a repeat of their porpoising issues from last season.
Verstappen topped the timing sheets with a fastest lap time of one-minute 14.606secs, ahead of Sergio Perez and Esteban Ocon in second and third respectively.
However, the potential return of porpoising issues, particularly for Mercedes, overshadowed the session.
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Russell took to the team radio early on during FP1 to complain of bouncing in the W14. Hearts will have sunk in the Mercedes garage as the message came through, with the team hopeful that the new upgrades introduced in Monaco might have come into their own this weekend.
However, Russell was not the only driver complaining of bouncing around the track, particularly heading into the final corner. Perez and Verstappen also struggled with the issue during FP1, with Lando Norris going as far to explicitly label the issue as "porpoising", not just bouncing.
Lewis Hamilton finished the session down in 12th - two places behind team-mate Russell in 10th - 1.239s behind Verstappen.
Nyck de Vries raised some eyebrows after he recorded the fourth fastest lap time of FP1, ahead of Pierre Gasly in fifth.
The teams will now have just a few hours to fine-tune and tweak their cars ahead of FP2, with Mercedes among those hoping that they can resolve the issue as quickly as possible.
Barcelona FP1 final results
1. Max Verstappen [Red Bull] - 1:14.606secs
2. Sergio Perez [Red Bull] - +0.768s
3. Esteban Ocon [Alpine] - +0.812s
4. Nyck de Vries [AlphaTauri] - +0.898s
5. Pierre Gasly [Alpine] - +0.939s
6. Fernando Alonso [Aston Martin] - +0.941s
7. Kevin Magnussen [Haas] - +1.083s
8. Charles Leclerc [Ferrari] - +1.088s
9. Carlos Sainz [Ferrari] - +1.120s
10. George Russell [Mercedes] - +1.147s
11. Lando Norris [McLaren] - +1.177s
12. Lewis Hamilton [Mercedes] - +1.239s
13. Zhou Guanyu [Alfa Romeo] - +1.300s
14. Yuki Tsunoda [AlphaTauri] - +1.309s
15. Lance Stroll [Aston Martin] - +1.333s
16. Valtteri Bottas [Alfa Romeo] - +1.372s
17. Oscar Piastri [McLaren] - +1.747s
18. Nico Hulkenberg [Haas] - + 1.855s
19. Logan Sargeant [Williams] - +1.900s
20. Alex Albon [Williams] - +2.024s
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