Lewis Hamilton was fastest for McLaren in the opening free practice session for the Canadian Grand Prix in Montreal.
A two-time winner in Montreal, the Briton edged out Sebastian Vettel's Red Bull by just over a tenth of a second, with Nico Rosberg's Mercedes third.
Lotus duo Kimi Raikkonen and Romain Grosjean exchanged the top spot initially, before Rosberg and Hamilton made it their own private battle for the rest of the session. First Rosberg put in a 1m16.837s 20 minutes in to depose Raikkonen, then Hamilton replied with a 1m16.080s seven minutes later.
The McLaren would stay in front for almost the rest of the session - deposed only for mere moments with 20 minutes to go when Rosberg did a 1m15.782s that Hamilton instantly answered with a 1m15.564s.
Rosberg was among several drivers who then tried super soft tyres in the closing stages, as with rain forecast for the afternoon, Mercedes, Red Bull, Lotus, Nico Hulkenberg, Daniel Ricciardo and the HRTs all chose to try the softer rubber earlier than usual. But the Mercedes did not go any quicker than it had on softs, so Hamilton retained first position, and Rosberg found himself pushed down to third right at the end when Vettel squeezed into second place on super softs.
Vettel may have to take a trip to the stewards' office between sessions, as he is under investigation for an incident where he took a shortcut over the final chicane to pass Bruno Senna's Williams and ensure clear track for the start of a flying lap.
Fernando Alonso was the quickest Ferrari driver in fourth, with team-mate Felipe Massa 12th as the pair evaluated different exhaust layouts.
Mark Webber completed the top five in the second Red Bull, following by Hulkenberg's Force India, the Saubers of Kamui Kobayashi and Sergio Perez, and Michael Schumacher's Mercedes.
Jenson Button was 10th despite his McLaren only managing 12 laps before it was sidelined in the garage with an oil leak.
Thirteen minutes of running were lost just after the mid-way point when Heikki Kovalainen put his Caterham into the wall on the exit of the Turn 8/9 chicane, doing substantial damage to the car's right-hand side. Kovalainen emerged unscathed. The rest of the field confined themselves to minor trips over the grass and asphalt run-offs.
Pos Driver Team Time Laps 1. Lewis Hamilton McLaren-Mercedes 1m15.564 30 2. Sebastian Vettel Red Bull-Renault 1m15.682s + 0.118 29 3. Nico Rosberg Mercedes 1m15.782s + 0.218 30 4. Fernando Alonso Ferrari 1m15.842s + 0.278 34 5. Mark Webber Red Bull-Renault 1m15.897s + 0.333 28 6. Nico Hulkenberg Force India-Mercedes 1m15.986s + 0.422 29 7. Kamui Kobayashi Sauber-Ferrari 1m16.000s + 0.436 31 8. Sergio Perez Sauber-Ferrari 1m16.249s + 0.685 32 9. Michael Schumacher Mercedes 1m16.264s + 0.700 28 10. Jenson Button McLaren-Mercedes 1m16.347s + 0.783 12 11. Paul di Resta Force India-Mercedes 1m16.460s + 0.896 32 12. Felipe Massa Ferrari 1m16.619s + 1.055 17 13. Pastor Maldonado Williams-Renault 1m16.859s + 1.295 25 14. Romain Grosjean Lotus-Renault 1m16.890s + 1.326 36 15. Kimi Raikkonen Lotus-Renault 1m17.014s + 1.450 41 16. Jean-Eric Vergne Toro Rosso-Ferrari 1m17.352s + 1.788 28 17. Daniel Ricciardo Toro Rosso-Ferrari 1m17.580s + 2.016 31 18. Vitaly Petrov Caterham-Renault 1m17.935s + 2.371 23 19. Heikki Kovalainen Caterham-Renault 1m18.177s + 2.613 16 20. Pedro de la Rosa HRT-Cosworth 1m18.182s + 2.618 26 21. Bruno Senna Williams-Renault 1m18.762s + 3.198 36 22. Narain Karthikeyan HRT-Cosworth 1m19.354s + 3.790 23 23. Timo Glock Marussia-Cosworth 1m20.004s + 4.440 21 24. Charles Pic Marussia-Cosworth 1m20.067s + 4.503 23 All Timing Unofficial