Lewis Hamilton sent Mercedes team mate and Formula One title rival Nico Rosberg a clear statement of intent with the fastest lap in first practice for the Mexican Grand Prix on Friday.
The triple world champion, 26 points behind overall leader Rosberg with three races remaining, lapped the Autódromo Hermanos Rodríguez with a best time of one minute 20.914 seconds.
Rosberg, winner of nine races to Hamilton's seven this season, was seventh on the timesheets and 0.759 off Hamilton's pace.
Mercedes said the championship leader had been running through his program as planned and had not suffered any problem.
Both he and Hamilton, who picked up where he left off after a dominant win in Texas last weekend, set their fastest laps on the medium compound tire before others switched to the quicker softs.
Hamilton needs to win on Sunday to boost his flagging title hopes, while Rosberg would be crowned champion if he wins and his team mate finishes 10th or lower.
Ferrari's Sebastian Vettel and Kimi Raikkonen were second and third on softs, with the German a mere 0.079 slower than the Briton on a cool and overcast morning at the high-altitude Mexico City circuit.
Mexican Sergio Pérez delighted the home crowd in fourth place ahead of Force India team mate Nico Hulkenberg and Williams' Valtteri Bottas.
The session was red-flagged when Sauber's Brazilian Felipe Nasr ran bumpily over a kerb, with the right section of his car's front wing shattering and sending a cloud of carbon fiber debris across the track.
Nasr had been testing a version of the 'halo' head protection device on his car.
Red Bull's Daniel Ricciardo, third in Texas behind the Mercedes pair, was eighth fastest with Dutch team mate Max Verstappen 14th.
Versatappen had a moment of drama when mechanics had to use fire extinguishers when he pitted with flames coming from his rear brakes. He then sat out the rest of the session.