Red Bull's Max Verstappen headed McLaren's Lando Norris in Friday practice at the Mexico City Grand Prix.
Verstappen was 0.119 seconds quicker than Norris, with Ferrari's Charles Leclerc 0.147secs adrift in third.
There were a couple of surprises just outside the top three - with Valtteri Bottas' Alfa Romeo fourth and Daniel Ricciardo's Alpha Tauri sixth.
Red Bull's Sergio Perez took fifth and Lewis Hamilton seventh in the Mercedes, struggling over one lap.
Hamilton, who had a strong performance at the US Grand Prix last weekend with an upgraded floor on the car, said: "A bit of a struggle in the car today. The car is night and day different compared to last week.
"You just never know what you're going to get with this one, some days she's great and some days she's not. It's hard to extract the lap.
"There is performance there it is just trying to extract it. It is quite peaky this weekend with the aero map or whatever it may be. It definitely wasn't a fun day compared to P1 in Austin."
But although Hamilton looked off the pace on his qualifying simulation run, the Mercedes was stronger on race pace, with Hamilton actually faster than Verstappen on average on his long run.
However, Verstappen set his fastest lap eight or so laps into his high-fuel run, a performance pattern not matched by his rivals, which suggests the Red Bull will again be the car to beat in the race.
Fastest of all was Norris, while the Ferraris typically were the slowest of the leading cars over a series of laps.
But after a close race between himself, Verstappen and Hamilton in Austin last weekend, Norris said he did not expect to be able to race with Verstappen on Sunday.
"It was a good Friday," he said, "a good start to the weekend. It's close. It can swing either way very quickly.
"I did a very good lap, maybe too good for this early in the weekend. We started the weekend off well. We generally always do. We have a good idea of where to put the car to begin with and it's not far from where we end the weekend.
"Still some stuff to improve but to challenge the Red Bull I think is going to be tough around here."
Verstappen said: "We look competitive so that's good but there are always a few things to improve but it was a positive start to the weekend, probably a little bit better than I expected.
"Tyres are very difficult to manage in the long run so there are still a few things to work on. It is going to be incredibly competitive over one lap."
Ricciardo, who had been disappointed with his performance in the race in Austin, said: "Today was fun. I do enjoy this place as well but I think the car was solid and we made good progress. We look like we are in a good place but it's very close. I'm very hungry and motivated and it feels good."
Behind Hamilton, Alpine's Esteban Ocon, McLaren's Oscar Piastri and Hamilton's team-mate George Russell completed the top 10, after missing the first session while Mercedes handed his car to young driver Frederik Vesti.
The one big incident of the session was a high-speed spin for Fernando Alonso in the middle of the Esses.
The Aston Martin driver caught it expertly, keeping the car away from the walls and popping a gear and getting going again as soon as the car was pointing in the right direction.
But the spin wrecked his soft tyres and the two-time champion ended the session last because he could not complete a lap time on the fastest rubber.
There were sprinkles of rain early in the session and as it came to an end, but not enough to stop the drivers from running on slick dry-weather tyres.
In the first session, in which Verstappen was also fastest, five teams used up one of their two mandatory young driver outings.
The fastest of them was Briton Oliver Bearman, a member of the Ferrari driver academy, who impressed in the Haas with 15th fastest time, four places and 0.4secs behind race driver Nico Hulkenberg.
Bearman had a scary moment when he got the car on the kerbs at the Esses and had to rescue what could have been a crash, but set a faster time on the soft tyres on his next lap.
Frenchman Isack Hadjar in the Alpha Tauri was 17th, 1.4secs slower than Ricciardo, ahead of Alpine's Jack Doohan and Vesti in the Mercedes.
Frenchman Theo Pourchaire had a torrid time in the Alfa Romeo, with a succession of braking problems that meant he failed even to complete a flying lap before his session was brought to a premature close.
