Robert Kubica maintained BMW-Sauber's impressive early season form with the quickest time on the opening day of testing at Sepang on Tuesday.
The Polish driver, who was forced to retire from the season opening Australian Grand Prix with gearbox problems, edged out world championship leader Kimi Raikkonen by just more than one tenth of a second.
Kubica's F1.07 car featured new aerodynamic solutions for the Sepang circuit, as well as modifications to the gearbox that the team hoped would overcome the problem he suffered at Albert Park.
Behind Raikkonen, Williams' Alexander Wurz showed improved form for his team - as the outfit spent time focusing on improving their single-lap pace and evaluating tyre options.
"The set-up is not good so that is what I worked on," Wurz told autosport.com.
"I tried two different sets of tyres, there is a big difference between the two so we have to see."
The Austrian also evaluated new Bridgestone tyre markings late in the day.
Rubens Barrichello showed some slightly better pace from Honda Racing after their disastrous Australian Grand Prix. The new RA107 was fitted with new front suspension components and their revised aero package, with the team trying to establish back-to-back tests to work out how much better the new parts are.
McLaren test driver Pedro de la Rosa, who hands over his car to Lewis Hamilton tomorrow, was fifth fastest, ahead of Toyota's Franck Montagny who brought out the only red flag of the day when his car stopped with suspected drivetrain problems just before lunch, but was able to rejoin and continue evaluating tyre compounds.
"We had a couple of problems which stopped me but in the afternoon we did some long runs with both sets," Montagny told autosport.com.
"I thought there was a big difference between the two but in the long run it was not so much, at least to us. The car was getting better and the track condition improved too."
Takuma Sato began work for Super Aguri to set the seventh fastest time but new aerodynamic parts due to be evaluated for the Malaysian Grand Prix only arrived late in the day so will not be run until tomorrow.
Behind eighth fastest David Coulthard, Nelson Piquet Jr. tried some minor engine modifications on the R27 but was compromised by mechanical issues that limited his mileage.
"Unfortunately, Nelson didn't manage to do much running today after we experienced some problems," said Renault's chief test engineer Christian Silk.
"Even so, we managed to get a decent baseline set-up for the car, and the balance is now good enough to continue our planned programme tomorrow with Heikki."
Toro Rosso's Scott Speed was the slowest out of the 10 cars present.
Tuesday's testing times:
Pos Driver Make Time Laps 1. Kubica BMW-Sauber (B) 1:36.187 92 2. Raikkonen Ferrari (B) 1:36.302 54 3. Wurz Williams-Toyota (B) 1:36.334 89 4. Barrichello Honda (B) 1:36.826 82 5. De la Rosa McLaren-Mercedes (B) 1:37.106 60 6. Montagny Toyota (B) 1:37.267 73 7. Sato Super Aguri-Honda (B) 1:37.389 72 8. Coulthard Red Bull-Renault (B) 1:37.609 82 9. Piquet Renault (B) 1:38.199 32 10. Speed Toro Rosso-Ferrari (B) 1:38.503 69 All timing unofficial