Sebastian Vettel led Mark Webber to a Red Bull one-two in second practice at the Indian Grand Prix.
Vettel was 0.599 seconds faster than his world championship rival Fernando Alonso of Ferrari, who clocked the third fastest time.
Mercedes' Nico Rosberg was fourth, ahead of Lotus's Kimi Raikkonen.
McLaren's Lewis Hamilton and Jenson Button seemed to be struggling in sixth and seventh places respectively, nearly a second slower than Vettel.
"The circuit was quite dusty," said Vettel. "It seemed to improve lap by lap. It depends on when you were on the track compared to the others, how many laps you have, but we can be quite happy.
"There was quite a lot of grip at the end and it just got better. I was surprised with the long runs. The lap times kept getting better.
"We didn't have any major issues. We were able to test a few things. Now we need to say whether we made progress, and hopefully we can make progress for tomorrow."
Alonso appears to be the closest thing to a challenge to Red Bull. Although he was significantly slower than Vettel, his tyres were several laps older than the German's when he recorded his time.
Ferrari have brought several new parts to this race as they seek to stem Red Bull's advance, including a modified front wing, brake ducts, rear wing and a revised diffuser.
Alonso lost the lead of the championship to Vettel at the last race in Korea, which was the Red Bull driver's third win in a row.
On the race-simulation runs later in the session, the Red Bulls and Alonso looked evenly matched.
Vettel set the fastest lap in race trim but it came after a slow lap that would have allowed the tyres to recover.
Alonso's fastest race-simulation lap was more than 0.3 secs slower but he had not given the tyres a rest beforehand.
The fastest race-run overall was Alonso's - his average lap time was 0.2 secs quicker than Vettel's, with Lotus's Kimi Raikkonen third fastest on average followed by Hamilton. But it is difficult to know how much to read into the times as the teams do not reveal their fuel loads.
Encouragingly for Ferrari, Alonso set his fastest times with all the new parts on his car - in the three previous races developments intended to improve the car were taken off for second practice after the team discovered they did not work.
BBC F1 technical analyst Gary Anderson said: "It was a very strong day on both low and high fuel from both the Red Bulls. It's a big gap back to Alonso - 0.6 secs - I don't think it will be that big tomorrow.
"But Red Bull know all about qualifying well. This new car spec is all about qualifying well, then you can get a good start, break the DRS gap in the first two laps and you're home and dry."
Hamilton was told by his engineer over team radio that he was the fastest driver on the high-fuel runs, but the overall lap times did not seem to bear that out.
The Ferrari looked a handful in low-fuel trim, with Alonso having a few 'moments' and team-mate Felipe Massa having two spins.