Sebastian Vettel put Ferrari into the Mexican Grand Prix mix with the fastest practice lap while Lewis Hamilton laid down a marker in his title battle with Mercedes teammate Nico Rosberg.
Red Bull's Australian driver Daniel Ricciardo, third in Texas behind the Mercedes pair, was eighth and fifth fastest respectively in the two practice sessions.
Triple champion Hamilton, 26 points behind Rosberg with three races remaining, had set the pace on a chilly and overcast morning at the Autodromo Hermanos Rodriguez with a lap of one minute 20.914 seconds.
Vettel ousted the Briton from the top in the afternoon, by just 0.004 of a second, with a lap of 1:19.790 on supersoft tyres that showed Ferrari are not about to let Sunday's race turn into another Mercedes duel.
It was the first time the Italian team had led a Formula One practice session since Belgium in August and Vettel was pumped up, calling McLaren's Fernando Alonso 'an idiot' after being slowed by the Spaniard.
"Maybe it was just me today, but I felt that I was actually in Mexico City, not on the race track, with a lot of traffic," said the four times champion.
"I think overall the pace was good, which was the more important thing. We can still improve for tomorrow, and then go from there."
A more competitive Ferrari could be good news for Hamilton if he can take the victory and have others finishing between him and Rosberg.
Rosberg, winner of nine races to Hamilton's seven this season, was seventh on the morning timesheets and 0.759 off Hamilton's pace but moved up to third in the afternoon -- albeit still 0.431 slower than his team mate.
Hamilton, who picked up where he left off after a dominant win in Texas last weekend, needs to win on Sunday to boost his flagging hopes of a fourth title while Rosberg would be crowned champion if he wins and his team mate finishes 10th or lower.
The Briton said the track felt a lot better than last year, when it returned after a 23-year absence, and he felt more comfortable in the car.
Mercedes have already clinched the constructors' championship for the third year in a row while Rosberg and Hamilton are the only drivers in contention for the other title.
Vettel's teammate Kimi Raikkonen, who was third in the morning, was fourth after lunch.
Mexican Sergio Perez kept the home crowd cheering with fourth place ahead of Force India team mate Nico Hulkenberg in the opening session but dropped back to 15th in the afternoon.
The morning 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 in a cloud of carbon fibre debris.
Red Bull's Dutchman Max Verstappen was 14th and seventh.
