Red Bull's Sebastian Vettel crashed out of practice in Montreal as Mercedes driver Nico Rosberg set the pace.
The world champion hit the barriers at the tricky final chicane at the Circuit Gilles Villeneuve on his first real flying lap.Ferrari's Fernando Alonso was second fastest with Rosberg's team-mate Michael Schumacher third.
McLaren's Jenson Button and Lewis Hamilton were fifth and sixth, with Paul di Resta eighth.
Vettel's accident came with half an hour to go in the 90-minute session, and left the world champion on the sidelines for the remainder of the morning.
After an early run on experimental medium compound tyres that will not be raced this weekend, Vettel lost control on his first flying lap on the soft tyres that are one of the two choices that must be raced on Sunday.
He headed into the chicane too quickly, ran wide, bounced over the kerbs and then crashed into the wall, writing off the right-hand side of his car.
Testing tyres
Continue reading the main story Nico Rosberg may have been fastest in first practice but he says he is worried about his Mercedes "destroying" the tyres during the race. There were 61 pit stops in Canada in 2010."I hit the kerb, slid off the track and hit the wall. I'm fine," he told his team over Vettel crashes in Canada practice the car-to-pit radio, before stalking grim-faced back to the garage.
BBC F1 pit-lane reporter Ted Kravitz said: "It is the second time in four races that he has done that - he also crashed in first practice in Turkey. But remember he then set pole and won the race, so it may not affect him too much."
Vettel's crash underlined one of the great challenges of Montreal's Circuit Gilles Villeneuve, where long straights are followed by tight chicanes bounded by concrete walls on a track with a dusty, low-grip, bumpy surface.
BBC 5 live analyst Maurice Hamilton said: "There's a huge challenge here - it's very easy to hook a wheel over the kerb and then launch yourself into the wall at many of the chicanes."
Vettel's team-mate Mark Webber also had a harmless trip over the grass at Turn Three, while it was a troubled session for Hamilton, Vettel's closest championship rival.
The Englishman lost all the data on his steering wheel read-out and returned to the pits to have the problem rectified.
But McLaren could not immediately fix it, and they sent Hamilton back out for the remainder of the session with no read-out.
That meant that he was changing gear from feel rather than with the light display fitted to all F1 cars to help the drivers know exactly the right moment to change gear for ultimate performance, and was also deprived of the ability to change the car settings that are adjustable from the cockpit.
Button also had problems when his DRS overtaking aid stopped working, which would have cost him about 8mph in straightline speed.
There were no such problems for Mercedes, for whom Rosberg underlined why Button had predicted the team would be a surprise competitive package in Canada.
The Mercedes has prodigious straight line speed thanks to a particularly effective DRS overtaking aid, and that gives them a 5mph advantage down the straights over most of their rivals.
On a track such as this, Button said that was a "massive advantage", and the 2009 world champion predicted his former team could be a threat for pole position on Saturday.
The Mercedes, though, has heavy tyre wear and fades quickly during races.
Ferrari may also be a team to watch here.
Fernando Alonso pushed Vettel for victory in Monaco, a track that despite its low-speed, requires similar things from a car to Montreal, and the team have some major upgrades to their car.
The Spaniard was close to the top of the times throughout the first session, finishing 0.548secs behind Rosberg.
Red Bull have dominated the season so far, with Vettel taking five wins and a second place from the six races so far.
But this is expected to be their toughest weekend of the year so far as the long straights and slow corners do not suit a car that lacks top-end power and excels in high-speed corners.
Nevertheless, Red Bull are confident they can be competitive and, like Ferrari, have a new front wing and diffuser here in a bid to keep their advantage over their rivals.
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