Sebastian Vettel led Mark Webber to a Red Bull one-two in second practice at the United States Grand Prix.
The German, crowned world champion two races ago, beat Webber by 0.115 seconds, with Mercedes' Nico Rosberg third from team-mate Lewis Hamilton.
Lotus's Heikki Kovalainen was fifth, from the Saubers of Esteban Gutierrez and Nico Hulkenberg and team-mate Romain Grosjean.
McLaren's Jenson Button was ninth ahead of Ferrari's Fernando Alonso.
Button has been given a three-place grid penalty for ignoring red flags in the first session on Friday morning in Austin.
Apparently struggling with his car, Hamilton made a series of mistakes in the early part of the session and ended up 0.173secs behind Rosberg.
He told his team he wanted to do another low-fuel qualifying simulation lap, but the team refused, telling him he had to get on with his high-fuel race-preparation run.
Hamilton said he had had "an OK day", adding: "The Red Bulls are a long way down the road but we'll still try to give them hell and catch them."
Vettel was fastest despite being blocked by Grosjean on his first attempt at a flying lap, and by a Williams on his second, so his tyres were not at their best when he set his fastest time.
The Red Bulls were also in a league of their own on the race-simulation runs in the final part of the session - Vettel was nearly a second a lap faster than the best driver from any other team, with Webber a few tenths of a second behind his team-mate.
BBC F1 technical analyst Gary Anderson said: "I think Vettel and Webber will be the front row but I think Mercedes will be keeping them honest. If a Red Bull makes a mistake a Mercedes could sneak onto the front row."
Arguably the most impressive performance of the session came from Kovalainen.
Drafted in by Lotus as a replacement for Kimi Raikkonen, who is missing the last two races of the season to have an operation on a long-standing back problem, he was competitive on both low and high fuel.
He was about 0.2secs quicker than team-mate Romain Grosjean on single-lap pace, although the Finn appeared to be lagging behind the Frenchman on race pace.
Lotus technical director Nick Chester said Kovalainen had done "a brilliant job".
With Red Bull already winners in both the drivers' and constructors' championships, Ferrari face a battle to get back the second place in the constructors' championship they lost to Mercedes a couple of races ago and just as much of a struggle to hold off Lotus, who are catching them in fourth place.
Ferrari also appeared to be lagging behind Sauber, in both low-fuel and high-fuel trim. The Italian team appear to have a difficult weekend ahead.