Max Verstappen topped the timesheets in the Mexican GP's first practice session, as Red Bull finished more than a second ahead of Mercedes and Ferrari.
Verstappen, a dominant winner of this race last year, led a Red Bull one-two and was impressively four tenths quicker than team-mate Daniel Ricciardo, while his 1:16.656 on hypersoft tyres was almost as quick as the 2017 pole time.
But Red Bull's surprise closest challengers in P1 were Renault's Carlos Sainz and Nico Hulkenberg, with Mercedes and Ferrari seeming to start the weekend with drastically different programmes.
Lewis Hamilton, who can win a fifth world championship in Mexico, was Mercedes' lead driver in fifth but he finished 1.4seconds off Verstappen's benchmark, and Valtteri Bottas was two tenths further back.
Ferrari, meanwhile, were more than two seconds off the pace as they tinkered with upgrades.
The Scuderia successfully reverted back to an old setup to win the US GP last weekend after their failed development attempts in Singapore, Russia and Japan, but trialled a new floor in Mexico.
Both Sebastian Vettel and Kimi Raikkonen tested the latest version, but didn't appear to be concentrating on one-lap pace - while the Finn span during the session.
Brendon Hartley was ninth for Toro Rosso despite using Honda's older-spec engine, though Pierre Gasly didn't post a time in the session using a newer power unit.
Gasly is taking engine penalties after Honda brought a new version with them to Mexico, but they are only using it for P1 to add to the Frenchman's pool to the season before he switches back to an old PU like Hartley.
Canadian youngster Nicholas Latifi impressively rounded off the top-10 for Force India, outpacing Sergio Perez after stepping in for Esteban Ocon.
And there were also strong showings from Sauber and McLaren stand-ins Antonio Giovinazzi and Lando Norris as they were similarly quicker than their temporary team-mates, Marcus Ericsson and Stoffel Vandoorne.
Norris has beaten Vandoorne, the man he is replacing at McLaren next season, in all three practice sessions they have taken part in together.
Williams once again look to be the backrunners in Mexico with Sergey Sirotkin and Lance Stroll propping up the timesheets.
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