BMW's Nick Heidfeld emerged fastest as the field scrambled to get some dry running at the end of a rain-affected final practice session at Spa-Francorchamps in Belgium.
McLaren duo Heikki Kovalainen and Lewis Hamilton ended up second and fourth, split by Fernando Alonso (Renault), while the Ferraris only managed fifth (Felipe Massa) and seventh (Kimi Raikkonen).
Morning rain meant the track was damp in places when the session began, and with steadily increasing drizzle greeting the drivers as they set out for their installation laps and heavier showers soon following, most preferred to get straight back to the pits.
However, with rain likely to be a factor later in the weekend, all bar the McLarens, Ferraris and BMW's Robert Kubica eventually chose to come out for some wet running when the circuit was at its most damp towards the middle of the session.
With the track changing all the time as the showers moved across the circuit, both Toro Rossos, both Red Bulls, Nico Rosberg (Williams) and Timo Glock (Toyota) all took turns at the top of the timing screens.
Glock and the two Williams drivers were the first to brave 'slick' tyres with 25 minutes remaining, they duly vaulted to the head of the order with times three seconds quicker than the intermediate runners. That prompted a flurry of quick laps and a wildly fluctuating leaderboard as the track then became faster and faster with every lap.
The title contenders eventually emerged in the final ten minutes, with Hamilton using the softer Bridgestones to go fastest by 1.3 seconds for a while. A wild slide on the exit of Les Combes cost him the chance to improve on the next lap.
Hamilton stayed in front until the final few minutes, when first Kovalainen edged ahead, then Heidfeld produced a 1:47.876 lap to end the session fastest by 0.289 seconds.
Alonso pushed Hamilton back to fourth with his final lap, with Massa eventually improving to fifth after languishing towards the foot of the top ten until his last run.
Toro Rosso's Sebastian Vettel split the Ferraris and broke into the top six again, while Rosberg, Nelson Piquet (Renault) and Mark Webber (Red Bull) completed the top ten behind Raikkonen.
Jenson Button was unable to get any running in either wet or dry conditions - his Honda expiring on his out-lap at the start of the session and parking at the side of the Kemmel Straight.
Pos Driver Team Time Laps
1. Heidfeld BMW Sauber (B) 1:47.876 19
2. Kovalainen McLaren-Mercedes (B) 1:48.165 + 0.289 10
3. Alonso Renault (B) 1:48.307 + 0.431 19
4. Hamilton McLaren-Mercedes (B) 1:48.356 + 0.480 10
5. Massa Ferrari (B) 1:48.692 + 0.816 6
6. Vettel Toro Rosso-Ferrari (B) 1:48.768 + 0.892 16
7. Raikkonen Ferrari (B) 1:48.815 + 0.939 10
8. Rosberg Williams-Toyota (B) 1:48.836 + 0.960 13
9. Piquet Renault (B) 1:48.946 + 1.070 17
10. Webber Red Bull-Renault (B) 1:49.054 + 1.178 14
11. Trulli Toyota (B) 1:49.057 + 1.181 16
12. Coulthard Red Bull-Renault (B) 1:49.125 + 1.249 11
13. Kubica BMW Sauber (B) 1:49.250 + 1.374 12
14. Bourdais Toro Rosso-Ferrari (B) 1:49.256 + 1.380 19
15. Glock Toyota (B) 1:49.535 + 1.659 18
16. Nakajima Williams-Toyota (B) 1:49.830 + 1.954 12
17. Fisichella Force India-Ferrari (B) 1:49.949 + 2.073 16
18. Sutil Force India-Ferrari (B) 1:50.034 + 2.158 16
19. Barrichello Honda (B) 1:50.061 + 2.185 17
20. Button Honda (B) No time
All timing unofficial