BMW's Nick Heidfeld was fastest in the final practice session for the Spanish Grand Prix.
The German leapt from 19th position to first when he switched to the softer tyres for his final run.
David Coulthard took second for Red Bull, ahead of Renault's Fernando Alonso.
Ferrari and McLaren appeared to continue their recent strategy of running heavier fuel loads in this practice session. Felipe Massa and Kimi Raikkonen were only ninth and 13th in the Ferraris, while McLaren teammates Lewis Hamilton and Heikki Kovalainen ended the morning 11th and 15th respectively.
A frenetic opening half-hour saw six drivers swapping first position, and as many as 16 cars covered by just one second.
Raikkonen was the first of the major contenders to complete a flying lap, and immediately went fastest on a 1:22.804, but within two minutes Kovalainen and BMW's Robert Kubica had both usurped the world champion.
Alonso was next to go fastest overall, to the delight of the home fans, and although his 1:22.634 was swiftly beaten by Rubens Barrichello (Honda), the Spaniard improved to a 1:22.271 on his next run to lead the way again.
Coulthard then moved ahead in the final 20 minutes, initially beating Alonso with a 1:22.068 lap and then improving to a 1:21.580 on his next outing - both times achieved on the harder Bridgestones.
The Scot remained on top until most of the field switched to soft tyres with four minutes remaining. He improved his own time to 1:21.465 on the soft compound, but this was not enough to beat Heidfeld and he had to settle for second.
Alonso jumped to third in the closing moments, with Kubica and Toyota's Jarno Trulli completing the top five.
Sebastien Bourdais was a surprise sixth for Toro Rosso, ahead of Alonso's Renault teammate Nelson Piquet and Honda driver Jenson Button.
Behind Massa, Timo Glock took the final top ten spot in the second Toyota.
Mark Webber failed to set a time in the second Red Bull. A suspected throttle problem caused the Australian's car to come to a halt in the pit exit before he could complete a flying lap.
As had been the case in Friday practice, the braking area for the tight Turn 10 was the scene of the session's main incidents, with both Barrichello and Force India's Adrian Sutil spinning down the asphalt run-off there before rejoining without drama.
Pos Driver Team Time Laps
1. Heidfeld BMW Sauber (B) 1:21.269 19
2. Coulthard Red Bull-Renault (B) 1:21.465 + 0.196 16
3. Alonso Renault (B) 1:21.599 + 0.330 16
4. Kubica BMW Sauber (B) 1:21.717 + 0.448 23
5. Trulli Toyota (B) 1:21.771 + 0.502 21
6. Bourdais Toro Rosso-Ferrari (B) 1:21.942 + 0.673 19
7. Piquet Renault (B) 1:21.992 + 0.723 18
8. Button Honda (B) 1:22.060 + 0.791 17
9. Massa Ferrari (B) 1:22.075 + 0.806 16
10. Glock Toyota (B) 1:22.081 + 0.812 23
11. Hamilton McLaren-Mercedes (B) 1:22.094 + 0.825 15
12. Rosberg Williams-Toyota (B) 1:22.174 + 0.905 19
13. Raikkonen Ferrari (B) 1:22.176 + 0.907 18
14. Nakajima Williams-Toyota (B) 1:22.189 + 0.920 16
15. Kovalainen McLaren-Mercedes (B) 1:22.220 + 0.951 16
16. Vettel Toro Rosso-Ferrari (B) 1:22.292 + 1.023 20
17. Barrichello Honda (B) 1:22.350 + 1.081 17
18. Fisichella Force India-Ferrari (B) 1:22.466 + 1.197 22
19. Sutil Force India-Ferrari (B) 1:22.689 + 1.420 21
20. Sato Super Aguri-Honda (B) 1:23.726 + 2.457 16
21. Davidson Super Aguri-Honda (B) 1:23.921 + 2.652 15
22. Webber Red Bull-Renault (B) 2
All timing unofficial