Nico Rosberg emerged fastest for Williams in the final practice session for the Canadian Grand Prix, as late incidents frustrated the teams' attempts to complete flying laps on the softer compound tyres.
Ferrari's Kimi Raikkonen took second behind Rosberg, with Lewis Hamilton third for McLaren and Raikkonen's teammate Felipe Massa in fourth.
After an uneventful first 45 minutes, the session was halted with 14 minutes to go when Sebastian Vettel crashed his Toro Rosso heavily at the exit of Turn 9.
The young German got out of the car unaided, but his incident left a significant amount of debris for the marshals to clear up, and only six minutes remained on the clock when the action resumed.
That led to a frantic dash on the super-soft tyres, as the entire field (bar Vettel) took to the track to attempt qualifying simulations as soon as the green light came on at the pit exit.
But they were interrupted again when Vettel's teammate Sebastien Bourdais also crashed - damaging his Toro Rosso's suspension on the Turn 5 wall and parking on the grass next to the circuit.
Although there was no need for another stoppage, the yellow flags for Bourdais' stranded machine prevented any late improvements.
That meant Rosberg remained on top, having been quickest throughout the second half of the session. He twice swapped the top spot with Raikkonen as both completed their first runs after 20 minutes, before settling at the head of the timing screens with a 1:16.555 lap - 0.034 seconds faster than Raikkonen's best.
Hamilton was 0.170 seconds slower than Rosberg in third, ahead of Massa.
Kazuki Nakajima jumped up to fifth in the second Williams just before the yellows for Bourdais' crash emerged, with Toyota's Jarno Trulli also improving at the same time to earn sixth.
Heikki Kovalainen was only seventh for McLaren, while Renault's Nelson Piquet was an encouraging eighth despite a quick spin at Turn 8 mid-way through the session and then another rotation at the final chicane on his last lap. He narrowly missed the wall and escaped damage.
Behind ninth-placed Mark Webber (Red Bull), Vettel was able to hang on to 10th place despite his accident.
Pos Driver Team Time Laps 1. Rosberg Williams-Toyota (B) 1:16.555 16 2. Raikkonen Ferrari (B) 1:16.589 + 0.034 22 3. Hamilton McLaren-Mercedes (B) 1:16.725 + 0.170 17 4. Massa Ferrari (B) 1:16.787 + 0.232 19 5. Nakajima Williams-Toyota (B) 1:16.898 + 0.343 16 6. Trulli Toyota (B) 1:16.946 + 0.391 20 7. Kovalainen McLaren-Mercedes (B) 1:16.952 + 0.397 16 8. Piquet Renault (B) 1:17.060 + 0.505 19 9. Webber Red Bull-Renault (B) 1:17.062 + 0.507 16 10. Vettel Toro Rosso-Ferrari (B) 1:17.085 + 0.530 16 11. Kubica BMW Sauber (B) 1:17.109 + 0.554 17 12. Bourdais Toro Rosso-Ferrari (B) 1:17.282 + 0.727 17 13. Coulthard Red Bull-Renault (B) 1:17.322 + 0.767 20 14. Barrichello Honda (B) 1:17.557 + 1.002 19 15. Glock Toyota (B) 1:17.557 + 1.002 20 16. Fisichella Force India-Ferrari (B) 1:17.608 + 1.053 24 17. Alonso Renault (B) 1:17.683 + 1.128 16 18. Heidfeld BMW Sauber (B) 1:17.766 + 1.211 17 19. Button Honda (B) 1:18.178 + 1.623 19 20. Sutil Force India-Ferrari (B) 1:18.424 + 1.869 21 All timing unofficial