Nico Rosberg recovered from an early pitlane error to top the final practice session for the Chinese Grand Prix.
The result means that the Williams driver has now been fastest in six of the nine practice sessions held so far in the 2009 Formula 1 season.
Jarno Trulli took second for Toyota, with Lewis Hamilton continuing to show improved form for McLaren in third, ahead of championship leader Jenson Button's Brawn GP.
Rosberg got his session off to an inauspicious start by sliding into the gravel in the pitlane entry at the end of his installation lap, repeating Lewis Hamilton's famous error from the 2007 race.
On this occasion the marshals removed the Williams from the gravel and allowed it to be brought back to the paddock, and just 20 minutes later Rosberg was back out on track and moving to the end of the timing screens with a 1:36.968 lap.
Prior to Rosberg's return, Kimi Raikkonen had been quickest for Ferrari, although the former champion spun at Turn 3 immediately after setting his benchmark time.
Nine minutes after losing the top spot to Rosberg, Raikkonen reclaimed it by 0.047s, although within moments the Ferrari had been beaten by Trulli's Toyota, as the Italian became the first to set a time on super soft tyres and moved 0.6s clear of the field.
It looked like Trulli's time would remain unbeatable as even Button lapped a tenth of a second slower on his super soft tyre run. But in the final minute Rosberg improved to 1:36.133, 0.139s faster than Trulli.
Hamilton then came within 0.058s of taking second from the Toyota as he pushed Button down to fourth place.
Renault's Nelson Piquet took a surprise fifth, ahead of Felipe Massa (Ferrari), Heikki Kovalainen (McLaren) and Kazuki Nakajima (Williams).
Timo Glock completed just six laps before a gearbox problem brought his Toyota back into the pits and ended his session. Work continued on the car throughout the rest of the hour, with the level of activity in the garage suggesting a gearbox change - and consequently a five-place grid penalty - was likely.
Both Red Bulls hit trouble too, as identical driveshaft issues halted Mark Webber and Sebastian Vettel after a handful of laps, forcing the team to make precautionary changes to avoid gearbox damage.
Fernando Alonso also spent much of the session in the garage with his Renault receiving mechanical attention, only completing six laps before his problems struck.
Pos Driver Team Time Laps
1. Rosberg Williams-Toyota (B) 1:36.133 17
2. Trulli Toyota (B) 1:36.272 + 0.139 22
3. Hamilton McLaren-Mercedes (B) 1:36.330 + 0.197 17
4. Button Brawn GP-Mercedes (B) 1:36.463 + 0.330 16
5. Piquet Renault (B) 1:36.464 + 0.331 16
6. Massa Ferrari (B) 1:36.528 + 0.395 18
7. Kovalainen McLaren-Mercedes (B) 1:36.547 + 0.414 13
8. Nakajima Williams-Toyota (B) 1:36.560 + 0.427 17
9. Raikkonen Ferrari (B) 1:36.568 + 0.435 17
10. Barrichello Brawn GP-Mercedes (B) 1:36.642 + 0.509 17
11. Heidfeld BMW Sauber (B) 1:36.702 + 0.569 14
12. Kubica BMW Sauber (B) 1:36.742 + 0.609 14
13. Buemi Toro Rosso-Ferrari (B) 1:36.742 + 0.609 16
14. Bourdais Toro Rosso-Ferrari (B) 1:36.834 + 0.701 17
15. Webber Red Bull-Renault (B) 1:37.330 + 1.197 8
16. Vettel Red Bull-Renault (B) 1:37.349 + 1.216 7
17. Sutil Force India-Mercedes (B) 1:37.534 + 1.401 17
18. Fisichella Force India-Mercedes (B) 1:37.732 + 1.599 18
19. Alonso Renault (B) 1:38.003 + 1.870 6
20. Glock Toyota (B) 1:39.110 + 2.977 6
All timing unofficial