World champion Sebastian Vettel set the pace in the opening practice for the Singapore Grand Prix as he and McLaren's Lewis Hamilton were separated by less than a tenth.
The two championship protagonists ended the first 90 minutes nearly a second faster than Jenson Button and Ferrari's points leader Fernando Alonso.
Pastor Maldonado was an encouraging fifth fastest for Williams ahead of Mark Webber in the second Red Bull.
Both Force India's impressed, with Nico Hulkenberg and Paul di Resta seventh and eighth ahead of Daniel Ricciardo's Toro Rosso and Sergio Perez's "unstably handling" Sauber.
Heavy early morning rain set the tone for the start of the session on the Marina Bay circuit, which was still significantly damp when free practice began. Unsurprisingly. it was a tentative first half-hour's running with only Jean-Eric Vergne, Romain Grosjean, Pedro de la Rosa and Timo Glock setting times on intermediates.
Alonso ended up being the fastest man on the grooved Pirellis with a 2m01.573s before Kamui Kobayashi reset the reference with a time three second faster on a set of soft slicks. Kobayashi improved his time further and stayed top through the middle of the session, heading a time sheet that showed Mercedes's Nico Rosberg and Michael Schumacher – sporting the vaunted updated W03s – in second and third.
It wasn't long before Rosberg moved ahead, but as the session moved toward the hour mark, and more slick running occurred through the field, so the times tumbled. In the final 30 minutes there was a fascinating period of running when first Hamilton, and then Vettel went fastest and the pair traded times at the top of the leaderboard.
Hamilton, who was one of several to clump over the curbs at Turn 10 heavily, then ended his running in the pits some 10 minutes before the end of the session. No one else looked likely to challenge Vettel's 1m50.566s, though it was during this period that Button moved ahead of Alonso to go third.
There were no significant yellows, though several drivers shot off down escape roads – most notably Maldonado and Grosjean, who was 15th fastest on his return, three places behind Lotus teammate Kimi Raikkonen.
Once again Hua Qing Ma did a steady job for HRT. On a less than grippy complicated street circuit the Chinese driver, standing in for Narain Karthikeyan, ended the session less than two seconds off the pace of Pedro de la Rosa on his first visit to the circuit.
Pos Driver Team Time Laps
1. Sebastian Vettel Red Bull-Renault 1m50.566 24
2. Lewis Hamilton McLaren-Mercedes 1m50.615s + 0.049 15
3. Jenson Button McLaren-Mercedes 1m51.459s + 0.893 17
4. Fernando Alonso Ferrari 1m51.525s + 0.959 20
5. Pastor Maldonado Williams-Renault 1m51.576s + 1.010 20
6. Mark Webber Red Bull-Renault 1m51.655s + 1.089 19
7. Nico Hulkenberg Force India-Mercedes 1m51.658s + 1.092 24
8. Paul di Resta Force India-Mercedes 1m51.943s + 1.377 21
9. Daniel Ricciardo Toro Rosso-Ferrari 1m52.275s + 1.709 24
10. Sergio Perez Sauber-Ferrari 1m52.296s + 1.730 19
11. Bruno Senna Williams-Renault 1m52.629s + 2.063 25
12. Kimi Raikkonen Lotus-Renault 1m52.716s + 2.150 16
13. Kamui Kobayashi Sauber-Ferrari 1m52.839s + 2.273 23
14. Michael Schumacher Mercedes 1m52.986s + 2.420 21
15. Romain Grosjean Lotus-Renault 1m53.028s + 2.462 20
16. Felipe Massa Ferrari 1m53.080s + 2.514 17
17. Jean-Eric Vergne Toro Rosso-Ferrari 1m53.189s + 2.623 25
18. Nico Rosberg Mercedes 1m53.227s + 2.661 25
19. Timo Glock Marussia-Cosworth 1m54.908s + 4.342 20
20. Heikki Kovalainen Caterham-Renault 1m55.091s + 4.525 16
21. Charles Pic Marussia-Cosworth 1m55.335s + 4.769 22
22. Vitaly Petrov Caterham-Renault 1m55.760s + 5.194 22
23. Pedro de la Rosa HRT-Cosworth 1m56.656s + 6.090 21
24. Ma Qing Hua HRT-Cosworth 1m58.053s + 7.487 20
All Timing Unofficial