Champion-elect Lewis Hamilton finished second to Mercedes team-mate Valtteri Bottas in opening practice for the Mexican Grand Prix.
Hamilton, who needs to finish only fifth on Sunday to secure his fourth Formula 1 World Championship, has been in sensational form since the summer break.
The British driver recorded his fifth victory in six races at the United States Grand Prix last Sunday to establish a 66-point lead over rival Sebastian Vettel with just three races remaining, but he trailed team-mate Bottas by nearly half-a-second on Friday morning.
Bottas, who has struggled to live with Hamilton in recent grands prix, dominated the 90-minute session and posted a best time of one minute and 17.824 seconds to finish 0.466secs clear of Hamilton with Red Bull's Max Verstappen in third.
Ferrari's Vettel, who must finish in the top two on Sunday to stand any chance of keeping his bleak title chances alive, was a distant fifth in the order, some 0.762secs shy of Bottas' best effort. Daniel Ricciardo was fourth for Red Bull.
Ferrari and Red Bull, unlike Mercedes however, did not post a lap on the ultrasoft tyre - the fastest of the three compounds available here this weekend - to suggest they may have more time in reserve.
Alfonso Celis, deputising for Esteban Ocon in front of his home crowd, was left red-faced after he lost control of his car at the final corner.
The 21-year-old thudded into the barrier and damaged the rear of his car. The session was temporarily red-flagged to remove his stricken Force India.
His mechanics will now face a race against time to ensure the car is available to Ocon for the day's second session.
Fernando Alonso, who will start Sunday's race from the back of the field after he was served a 20-place penalty following yet more problems with his McLaren-Honda engine, was eighth quickest. His team-mate Stoffel Vandoorne failed to even set a time in opening practice following an engine glitch.
Elsewhere, Kimi Raikkonen was sixth for Ferrari ahead of Mexican Sergio Perez, while Felipe Massa finished ninth for Williams and Renault's Nico Hulkenberg completed the top 10.
Practice One timesheet
1. Valtteri Bottas Mercedes 1:17.824
2. Lewis Hamilton Mercedes +0.466
3. Max Verstappen Red Bull +0.571
4. Daniel Ricciardo Red Bull +0.597
5. Sebastian Vettel Ferrari +0.762
6. Kimi Raikkonen Ferrari +1.184
7. Sergio Perez Force India +1.416
8. Fernando Alonso McLaren +1.522
9. Felipe Massa Williams +1.619
10. Nico Hulkenberg Renault +1.728
11. Carlos Sainz Renault +1.730
12. Lance Stroll Williams +1.948
13. Kevin Magnussen Haas +2.820
14. Pascal Wehrlein Sauber +3.147
15. Antonio Giovinazzi Haas +3.445
16. Charles Leclerc Sauber +3.622
17. Sean Gelael Toro Rosso +3.815
18. Brendon Hartley Toro Rosso +3.923
19. Alfonso Celis Force India +4.518
20. Stoffel Vandoorne McLaren No time
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