Nico Rosberg ended the first day of Malaysian Grand Prix practice fastest for Mercedes, continuing his strong run of form at the start of the 2014 Formula 1 season.
The Australian Grand Prix winner was among the first of the frontrunners to switch onto the faster medium-compound tyre.
He knocked Felipe Massa's Williams, which had been the first to top the timesheets on the quicker rubber, off top spot shortly after the 30-minute mark.
Rosberg's Mercedes team-mate Lewis Hamilton went second fastest, a tenth-and-a-half slower than Rosberg, shortly afterwards but appeared to have the pace to go quicker but for a slow middle sector.
The Mercedes were then split by the back-on-form Kimi Raikkonen, who was only 0.035s slower than Rosberg as he went second for Ferrari.
This was just enough to retain second ahead of Sebastian Vettel, whose Red Bull set its fastest time on his second flying lap on medium rubber, which suggests the RB10 is proving to be kind on its tyres in the heat of Malaysia.
Fernando Alonso ended up fifth fastest, half a tenth slower than Hamilton and just eight thousandths of a second quicker than Massa.
Alonso had earlier set the pace when the field was running on fresh hard rubber, his earlier lap of 1m40.737s good enough to shade Hamilton on the Mercedes driver's best mark on the slower Pirellis.
Jenson Button, driving the upgraded McLaren that includes a modified nose as part of a significant update package, was eighth fastest behind Daniel Ricciardo's Red Bull, which suffered a brief off late in the session, and ahead of Bottas in the second Williams.
But Button's team-mate had a more difficult day. Kevin Magnussen was forced to stop during the morning session when an exhaust temperature sensor warning kicked in and he completed only a couple of installation laps early on in practice two before going straight into his medium-tyre running and ending up 12th fastest.
The session was characterised by plenty of spectacular moments as drivers struggled for rear-end grip.
Toro Rosso's Jean-Eric Vergne suffered a spin at the final corner and Force India's Nico Hulkenberg locked up and ran through the gravel at the exit of the same turn, later setting a time good enough to beat the Frenchman into the top 10.
Marussia driver Max Chilton also suffered a spin at Turn 3 early in the session. He was unable to get the car going again, but was recovered to the pits and returned to the track later on.
Lotus endured another difficult session. Pastor Maldonado did not run at all because of what is believed to be a turbo problem that developed in morning practice.
Team-mate Romain Grosjean did take to the track, but his running was first interrupted and then ended prematurely by a gearbox problem.
Caterham driver Kamui Kobayashi was unable to run because of an energy store problem that cropped up during morning practice and a subsequent gearbox issue, although team-mate Marcus Ericsson was able to complete a full programme.
Pos Driver Team/Car Time Gap Laps 1. Nico Rosberg Mercedes 1m39.909s 30 2. Kimi Raikkonen Ferrari 1m39.944s +0.035s 30 3. Sebastian Vettel Red Bull-Renault 1m39.970s +0.061s 30 4. Lewis Hamilton Mercedes 1m40.051s +0.142s 32 5. Fernando Alonso Ferrari 1m40.103s +0.194s 29 6. Felipe Massa Williams-Mercedes 1m40.112s +0.203s 34 7. Daniel Ricciardo Red Bull-Renault 1m40.276s +0.367s 29 8. Jenson Button McLaren-Mercedes 1m40.628s +0.719s 28 9. Valtteri Bottas Williams-Mercedes 1m40.638s +0.729s 35 10. Nico Hulkenberg Force India-Mercedes 1m40.691s +0.782s 34 11. Jean-Eric Vergne Toro Rosso-Renault 1m40.777s +0.868s 33 12. Kevin Magnussen McLaren-Mercedes 1m41.014s +1.105s 20 13. Adrian Sutil Sauber-Ferrari 1m41.257s +1.348s 28 14. Daniil Kvyat Toro Rosso-Renault 1m41.325s +1.416s 32 15. Esteban Gutierrez Sauber-Ferrari 1m41.407s +1.498s 34 16. Sergio Perez Force India-Mercedes 1m41.671s +1.762s 25 17. Romain Grosjean Lotus-Renault 1m42.531s +2.622s 14 18. Max Chilton Marussia-Ferrari 1m43.638s +3.729s 20 19. Jules Bianchi Marussia-Ferrari 1m43.752s +3.843s 29 20. Marcus Ericsson Caterham-Renault 1m45.703s +5.794s 31 21. Kamui Kobayashi Caterham-Renault 0 22. Pastor Maldonado Lotus-Renault 0