Mark Webber set the fastest time of final free practice ahead of this weekend's Belgian Grand Prix.
The Saturday morning session was affected by rain, as were both yesterday, beginning on a damp track, drying during the middle of the hour before a torrential downpour in the last 15 minutes.
It was 15 minutes into the hour before the first cars ventured out onto track on slick tires.
Several drivers took a turn at the top as the times fell, including Sebastian Vettel, Adrian Sutil, Nico Hulkenberg, Robert Kubica and Lewis Hamilton.
Conditions were good when Webber set his 1m46.106s lap - nearly three seconds quicker than the best time yesterday. Shortly before that, his Red Bull team-mate Vettel had been able to take Eau Rouge flat out with only one hand on the steering wheel while he covered the RB6's F-duct.
Vettel's session ended early when his car cut out and stuck in gear at the pit lane entry. He was pushed back to his box, where the team examined the problem. His engine was re-fired with 12 minutes to go but he did not return to the track. Red Bull put his incident down to an air box problem and say he was told to shut down the engine as a precaution.
Hamilton, Vettel and Jenson Button completed a sweep of the first four places for Red Bull and McLaren, with Kubica fifth and Fernando Alonso sixth, after a big sideways moment at Pouhon mid-way through the hour.
Felipe Massa, Sutil, Hulkenberg and Kamui Kobayashi completed the top 10.
The heavy rain in the last 15 minutes caught out Jarno Trulli and Hamilton, as both arrived at the Bus Stop chicane, on slick tires, to find a sodden braking zone.
As the rain increased, only Jaime Alguersuari and Michael Schumacher went out to complete several laps - with Schumacher sliding off the road at Eau Rouge on his out lap. Button joined them on circuit late on, while the likes of Hamilton and Alonso were out of their cars for the last 10 minutes.
Pos Driver Team Time Gap Laps
1. Mark Webber Red Bull-Renault 1m46.106s 10
2. Lewis Hamilton McLaren-Mercedes 1m46.223s +0.117 17
3. Sebastian Vettel Red Bull-Renault 1m46.396s +0.290 12
4. Jenson Button McLaren-Mercedes 1m46.397s +0.291 17
5. Robert Kubica Renault 1m46.492s +0.386 17
6. Fernando Alonso Ferrari 1m46.627s +0.521 11
7. Felipe Massa Ferrari 1m46.962s +0.856 10
8. Adrian Sutil Force India-Mercedes 1m47.064s +0.958 15
9. Nico Hülkenberg Williams-Cosworth 1m47.160s +1.054 16
10. Kamui Kobayashi Sauber-Ferrari 1m47.296s +1.190 17
11. Pedro de la Rosa Sauber-Ferrari 1m47.388s +1.282 15
12. Vitaly Petrov Renault 1m47.406s +1.300 15
13. R.Barrichello Williams-Cosworth 1m47.512s +1.406 14
14. M.Schumacher Mercedes 1m47.695s +1.589 16
15. Nico Rosberg Mercedes 1m47.837s +1.731 9
16. Sébastien Buemi Toro Rosso-Ferrari 1m47.905s +1.799 20
17. J.Alguersuari Toro Rosso-Ferrari 1m47.981s +1.875 18
18. Vitantonio Liuzzi Force India-Mercedes 1m48.692s +2.586 8
19. Jarno Trulli Lotus-Cosworth 1m50.600s +4.494 14
20. Bruno Senna HRT-Cosworth 1m51.133s +5.027 9
21. H.Kovalainen Lotus-Cosworth 1m51.384s +5.278 14
22. Lucas Di Grassi Virgin-Cosworth 1m51.517s +5.411 13
23. Timo Glock Virgin-Cosworth 1m51.669s +5.563 10
24. Sakon Yamamoto HRT-Cosworth 1m52.001s +5.895 13