Sebastian Vettel headed Mark Webber in a Red Bull one-two ahead of Ferrari's Fernando Alonso in second practice at the Korean Grand Prix.
The world champion was 0.032 seconds quicker than his team-mate while Alonso, who sneaked ahead of McLaren's Jenson Button, was 0.328secs behind.
Mercedes' Michael Schumacher and Nico Rosberg in fifth and seventh places sandwiched Ferrari's Felipe Massa.
Lewis Hamilton was eighth after not completing a lap on the faster tyres.
Hamilton had been fastest in the first session but was 0.885secs slower than Vettel in the second.
The 27-year-old ran competitively early on, setting times very similar to those of the Red Bulls and Ferraris, but hit traffic when the other front-runners did their qualifying-simulation runs in the middle of the session.
Hamilton said: "[The first practice] was pretty good for us, I was quite comfortable. So I've got to try to figure out what was going on."
A week after Vettel dominated the Japanese Grand Prix to close to just four points behind leader Alonso in the championship, the Red Bulls continued to show ominous form.
Vettel said: "This morning it was better than we expected. This afternoon it was quite slippery to start with but then it got a little bit better. All in all the car felt all right and now we have to see what we can do for tomorrow.
"We have to improve ourselves to match the others."
However, Button was told by his engineer during the race-simulation runs that he was the fastest car on the track.
The lap times did not quite bear this out - Button did the fastest lap on high fuel by 0.029secs from Alonso but the fastest run-average was set by the Ferrari number one.
The world championship leader's average on his race run was one minute 45.588 seconds, compared with Button's 1:45.797, Vettel's 1:46.172 and Raikkonen on 1:46.368. Hamilton did not do a representative set of laps.
And the Ferrari looked more competitive over one lap than it has often done this season, when its strength on race pace has not generally been matched in qualifying.
Button said: "It was a positive day, we're still not quick enough but the feeling with the car is reasonable and we made progress. [The long run] was all right but there is still room for improvement."
Both Alonso's attempts to set a quick time on the faster 'super-soft' tyres were hampered by coming up behind Pedro de la Rosa's HRT.
And the veteran Spaniard, who was testing a new rear wing for the back-of-the-grid team, incurred the wrath of Schumacher for holding him up on a quick lap, too.
But the stewards found Schumacher to be the guilty party - and have given him a reprimand for blocking De la Rosa.
Lotus driver Kimi Raikkonen ended the session 10th fastest, behind the Force India of Nico Hulkenberg, while testing a new exhaust system that the team hope will allow him to revitalise his championship hopes.
Raikkonen's team-mate Romain Grosjean, who does not have the new exhaust, was just 0.118secs behind the Finn in 11th place.
BBC F1 technical analyst Gary Anderson said: "I think Lewis is a bit disgruntled - he didn't get a time in on the super-softs and he was complaining about his front tyres going off during the race runs.
"Red Bull look pretty strong. Jenson Button looked good on the race simulations - he was probably the fastest car out there.
"But they're all up there, the Red Bulls, Ferraris and McLarens. And it looks like Schumacher and Rosberg might be getting in there a bit in among the title contenders."