Lewis Hamilton made a good start to a Mexican GP weekend at which he can clinch a sixth world title by posting the fastest time in first practice.
The Mercedes driver topped a staccato session by 0.119 seconds from Ferrari's Charles Leclerc, who was using harder, slower tyres.
Hamilton needs to out-score team-mate Valtteri Bottas by 14 points to seal his sixth world title on Sunday.
The Finn ended the session fifth, 0.678secs behind his team-mate.
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Behind Leclerc, Red Bull's Max Verstappen was third, just 0.015secs off Leclerc, and ahead of his team-mate Alexander Albon.
Ferrari's Sebastian Vettel was sixth fastest, 0.891secs off the pace, after a number of off-track moments caused by locking his brakes into Turn One at the end of the long straight, which sees the fastest straight-line speeds of the year.
The session started slowly, because the track was damp after overnight rain, and 25 minutes went by before anyone set a lap time.
And it was interrupted by a red flag for a few minutes mid-session when Lance Stroll crashed his Racing Point at the final corner.
When cars were running, there were several incidents when front-runners were tripped up by traffic in the tight and twisty final sector of the lap, which winds through a baseball stadium.
And the drivers were battling with low grip, partly caused by the dirty track surface, and partly by the thin air in Mexico City, where the Autodromo Hermanos Rodriguez is located more than 2,000m above sea level.
That means that teams struggle with a lack of aerodynamic downforce, because the cars cannot generate as much in the thin air, as well as engine and brake cooling.
The headline lap times are hard to read - not only was Leclerc running on harder tyres, which suggests his lap was the de facto fastest of the session, but Albon and Bottas, who were 0.6secs off the pace, set their best lap times before the red flag and the others after it, when the track would be in better condition.
Best of the rest behind the big three was McLaren's Carlos Sainz, with Toro Rosso's Pierre Gasly and Daniil Kvyat next, ahead of the Alfa Romeo of Antonio Giovanizzi.