Malaysian GP: Nico Rosberg fastest, Lewis Hamilton fourth
Mercedes driver Nico Rosberg beat Ferrari's Kimi Raikkonen to set the fastest time in second practice at the Malaysian Grand Prix.
Red Bull's Sebastian Vettel was third as the fastest three cars were split by just 0.061 seconds.
Rosberg's team-mate Lewis Hamilton was fourth from Ferrari's Fernando Alonso, both within 0.2secs of Rosberg.
Williams' Felipe Massa was sixth, from Red Bull's Daniel Ricciardo and McLaren's Jenson Button.
The second Williams of Valtteri Bottas and Force India's Nico Hulkenberg completed the top 10, from Toro Rosso's Jean-Eric Vergne and McLaren's Kevin Magnussen.
Rosberg was just 0.035secs quicker than Raikkonen, while Vettel's lap was impressive as it came on the second lap on the faster 'medium' tyres.
"We are more or less there in terms of pace, which is very good news," said Vettel. "For everything else we have to be patient. It is still early days, we still have a lot of problems we need to solve.
"But when we get the package together we seem to be on the right page, so I'm looking forward to tomorrow and Sunday."
BBC F1 analyst Allan McNish pointed out that Hamilton and Alonso both appeared to have more potential than they showed.
"They were both 0.5secs off the pace in the first sector of the lap," McNish said. "If they put a lap together, they look like they have more in their cars than their team-mates."
"We're just focusing on ourselves," said Hamilton. "You're guess is as good as mine [how everyone stands]. The car feels quite good. On the longer run it is pretty tough but I think everyone is in the same boat."
McNish added that he believed Mercedes may face a threat from Red Bull.
"For me, it's very close and competitive on single-lap performance and very close on the long runs. This is not a foregone conclusion. Mercedes look very good, and we don't know how much fuel they had on board to set those times, but Vettel and Red Bull are looking a lot more competitive than they did in Melbourne," he said.
An analysis of the times when the teams were doing race preparation suggests Mercedes have an advantage over Red Bull of about 0.4secs a lap, with Ferrari and Force India another 0.6secs further adrift.
Williams do not appear as competitive as they did in Australia two weeks ago and Button said McLaren were struggling.
The teams had to make changes to their cars to cope with the high ambient temperatures in tropical Malaysia - Mercedes and Ferrari both had increased cooling in their engine covers.
McLaren have a new front nose to their car, with a higher top to the bodywork behind the forward 'anteater' section, aimed at getting more airflow under the car and increasing downforce.
However, the change did not appear to deliver the half-second improvement chairman Ron Dennis had predicted in Australia.
Magnussen, who finished a sensational second to Rosberg in his debut race in Australia two weeks ago, had a difficult day.
His car ground to a halt in the first session when a sensor put the car into 'limp mode' to protect the engine. And in the second session he complained of not being able to get the power down and the team made changes to both the front suspension and brakes.
The Dane's problems paled compared to those of Lotus, however.
Still trying to catch up after a late start to pre-season testing, the car's reliability is poor.
Pastor Maldonado failed to set a time in either of the sessions following an engine problem and while Romain Grosjean managed 14 laps in the second session, he was 2.6 seconds off the pace and ended his day early when the car ground to a halt out on the track.