Lewis Hamilton controlled the second Thursday practice session in Monte Carlo, leading the way for the majority of the hour.
The McLaren driver topped the times early on, then was briefly usurped by Nico Rosberg when the Williams switched to soft tyres halfway through the session.
But Rosberg only stayed at the head of the pack for ten minutes before Hamilton too went for the softer Bridgestones and improved to 1:15.140, beating the Williams' time by just under four tenths of a second.
Rosberg was able to hang on to second place however, ahead of Ferrari duo Kimi Raikkonen and Felipe Massa, with Heikki Kovalainen fifth in the second McLaren.
Both Renaults were damaged in almost identical crashes at Sainte Devote. First the struggling Nelson Piquet spun into the barriers and deranged his rear wing, then his teammate Fernando Alonso lost control at the same spot 12 minutes later.
Unlike Piquet, Alonso managed to keep the car moving, but as he slid wildly sideways, his left-rear corner tagged the barriers, damaging the suspension and wing.
Although he successfully hauled the Renault back around to the pits, Alonso deposited plenty of debris along the way, forcing a four-minute red flag so the marshals could tidy up behind him.
While several other drivers had to short-cut the chicane, the only other men to actually strike the barriers were Jarno Trulli and Adrian Sutil. Trulli scraped his Toyota down the wall after a rough flight over the kerbs on the exit of the Swimming Pool, while Sutil lost his front wing on the Rascasse guard rail in the final minutes.
Despite his accident, Alonso managed to repeat his seventh place from the morning, just behind BMW's Robert Kubica.
The two Hondas took eighth and tenth - with Jenson Button a scant 0.067 seconds faster than Rubens Barrichello - as Kazuki Nakajima edged the second Williams between them.
Pos Driver Team Time Laps
1. Hamilton McLaren-Mercedes (B) 1:15.140 40
2. Rosberg Williams-Toyota (B) 1:15.533 + 0.393 39
3. Raikkonen Ferrari (B) 1:15.572 + 0.432 42
4. Massa Ferrari (B) 1:15.869 + 0.729 37
5. Kovalainen McLaren-Mercedes (B) 1:15.881 + 0.741 39
6. Kubica BMW Sauber (B) 1:16.296 + 1.156 34
7. Alonso Renault (B) 1:16.310 + 1.170 27
8. Button Honda (B) 1:16.351 + 1.211 45
9. Nakajima Williams-Toyota (B) 1:16.372 + 1.232 40
10. Barrichello Honda (B) 1:16.418 + 1.278 32
11. Heidfeld BMW Sauber (B) 1:16.426 + 1.286 44
12. Glock Toyota (B) 1:16.688 + 1.548 46
13. Webber Red Bull-Renault (B) 1:17.094 + 1.954 39
14. Coulthard Red Bull-Renault (B) 1:17.131 + 1.991 39
15. Piquet Renault (B) 1:17.246 + 2.106 35
16. Fisichella Force India-Ferrari (B) 1:17.251 + 2.111 33
17. Trulli Toyota (B) 1:17.379 + 2.239 28
18. Bourdais Toro Rosso-Ferrari (B) 1:17.581 + 2.441 38
19. Sutil Force India-Ferrari (B) 1:18.176 + 3.036 31
20. Vettel Toro Rosso-Ferrari (B) 1:18.225 + 3.085 38