Red Bull's Sebastian Vettel set a searing pace to top second practice at the Singapore Grand Prix.
The world champion beat team-mate Mark Webber by 0.604 seconds, and was a second faster than the first driver not in a Red Bull, Mercedes' Nico Rosberg.
The German was 1.009secs slower than Vettel and 0.11secs quicker than team-mate Lewis Hamilton.
Lotus's Romain Grosjean was fifth, ahead of Ferrari's Fernando Alonso, and the McLaren of Jenson Button.
Lotus's Kimi Raikkonen was eighth fastest ahead of Force India's Adrian Sutil and the second McLaren of Sergio Perez.
Vettel said: "I'm a bit surprised [to be so far ahead]. I was a bit surprised by how much I could improve on the soft tyre compared to the medium.
"I think it is not completely representative. We're quick but I think Mercedes will be very competitive in qualifying."
Alonso was 1.442secs slower than Vettel and appears to have no chance of beating his title rival on pace this weekend, despite a series of upgrades to his car.
Ferrari have brought new front and rear wings and a new design of diffuser to Singapore as they attempt to enable Alonso to close the 53-point gap to Vettel.
But the car appeared to lack grip throughout Friday practice, with Alonso visibly sliding the rear of the car on the exit of a number of corners.
"We can't say this was a good start, because we weren't competitive," Alonso said. "But if we consider that last year, at the end of a difficult weekend, we managed to get on the podium, we definitely aren't throwing in the towel yet.
"We have yet to analyse the data, but we have to be realistic, because when you have one car in sixth place and the other in 15th, it's clearly not the result we were hoping for, without taking into consideration the updates we have brought for this race."
Webber had been quicker than Vettel in both the first session and after the first runs of the second session.
But the Australian hit the wall on the exit of Turn 21 on his fastest lap, costing himself a few tenths of a second.
Hamilton had set the pace from Webber in the first session, but even then the Red Bull appeared to be a faster car than the Mercedes.
And in the second session Vettel and Webber were in a league of their own both as the teams tried out their cars in the trim in which they will dispute grid positions in qualifying on Saturday, and when they did longer runs on race fuel loads later in the session.
Hamilton said: "We just went on with our programme and the others looked a bit too quick. The balance is not very good. In P1 it was not too bad but in P2 it was terrible. That's how it always is in P2 and I'm sure tomorrow will be different.
"It's important to be on the front row at a street circuit. We're looking to try to get ourselves on the front row but it's not looking that good at the moment."
Triple world champion Sir Jackie Stewart, in his role as BBC 5 live sports extra analyst, said: "Red Bull have shown that they are dominant - to have both guys up there first and second and then the Mercedes third and fourth demonstrates they are a bit ahead.
"Grosjean is nearly a second and a half slower and that is not acceptable. The third and fourth teams have to get their acts together or we will again see the Red Bulls disappearing in the early laps of the race."
The session was relatively incident-free, apart from an action-packed 90 minutes for Williams's Pastor Maldonado, who had several off-track moments, breaking a front wing at Turn 13 in one of them.