Ferrari's Fernando Alonso set the pace in a crash-strewn practice session at the Canadian Grand Prix.
Alonso beat Red Bull's Sebastian Vettel by 0.369 seconds, with Ferrari's Felipe Massa third ahead of the McLaren's Lewis Hamilton and Jenson Button.
The session was disrupted by three crashes, two of which caused red flags while the track was cleared.
Force India's Paul di Resta was sixth on his first time at the track, ahead of Red Bull's Mark Webber.
The tone for an incident-packed practice was set in the morning when Vettel lost control of his Red Bull at the renowned "wall of champions", at the exit of the final chicane.
There were more accidents to come in the afternoon session when Force India's Adrian Sutil and Sauber's Kamui Kobayashi crashed within a few minutes of each other.
Sutil slid wide into the wall on the exit of the chicane at Turns Six and Seven, tearing the left front wheel off his car. He managed to park the car behind a barrier, ensuring the session could carry on.
But a few minutes later, Kobayashi lost control through the chicane at Turns Three and Four and crashed into the wall on the outside of the corner.
The car came to rest in the middle of the track, forcing the session to be stopped.
Virgin reserve driver Robert Wickens, standing in as an analyst for BBC 5 live, said: "It just seemed like he got a little bit too much kerb there and that bottomed out the car and took the front wheels off the ground."
The session was stopped for 10 minutes while the debris was cleared, but it was stopped again only three minutes after the restart when D'Ambrosio had an almost identical crash to Kobayashi's.
Vettel's car was repaired in time for the start of the second session and he was the early pace-setter.
But Alonso beat Vettel's time about halfway through the session, not long before Sutil's crash.
The Ferrari driver then went quicker again when the drivers finally got some running on the super-soft tyres in the final few minutes of the session, leaving the fastest lap at one minute 15.107 seconds.
BBC 5 live analyst Maurice Hamilton said: "This is where Ferrari hoped to be, this is a circuit where downforce is not quite so important, and that is where their weakness lies. The other thing in their favour is that the car does not work very well on the hard tyre, and that is not being used here."
When it was re-started again, Alonso, who was already fastest, set a new best time, before surviving a scary moment when he ran wide on to the grass, before recovering the moment and continuing.
Asked about it by BBC Sport, he smiled and flicked his backside sideways, to indicate that the rear of the car had stepped away from him.
He said: "We push a little bit more, at the end of the session. You try to be a little bit close to the walls and take a little bit more kerbs. This is Canada, so any little mistake puts you in the wall.
"We know the combination of the old Pirelli which is not easy to drive when they degrade plus the Canada circuit is a good combination for you guys to watch and a little bit more risky for us."
Alonso downplayed the importance of being fastest on Friday.
"We didn't concentrate on the times or whatever, so it is always positive to see yourself in the top four or five positions in practice but to be honest today is not important.
"We also topped the practice in Monaco and then in qualifying Vettel was one second quicker than us.
"We know that tomorrow is the day to put everything and we know Red Bull will be very strong and probably the favourites and also rain is forecast."
Hamilton also suffered from the accidents, picking up a puncture and limping back to the pits with damaged rear bodywork.
It was another blow in a difficult day for Hamilton, who had to drive much of the first session with no cockpit display after an electronics problem in his steering column, although he remained upbeat about the progress he had made, saying he had had a "good day".
"We got through a lot of testing but I'm just trying to find the exact balance," he said. "I've not optimised my set-up so I'll sit down tonight to dry to dial the car in.
"It's very slippery, the grip level is not spectacular and it's bumpy out there. We've done a heavy fuel run and it feels reasonably competitive. We might have good race pace.
"I'm sure the Red Bulls will pull something out in qualifying but we'll be there to put some pressure on them. I think the pace is there. I think it looks very close between the top drivers - Red Bull, Ferrari and Jenson are very fast."
Button added: "We've done some high fuel stuff on the softer tyre, and that was key today, because tomorrow we won't be running with 140kg (of fuel). We don't know what the car is like with low fuel on the super-soft because we didn't get a chance to do that. I think we'll be a little bit closer than we have been in the last few races."
But for Sauber, Kobayashi's crash merely compounded a difficult afternoon as their other driver Sergio Perez was forced to withdraw from the weekend after feeling unwell in first practice following his crash in Monaco two weeks ago, which left him with concussion.
Perez has been replaced by Spaniard Pedro de la Rosa, who drove for Sauber for much of last season.
De la Rosa, the McLaren reserve driver, is significantly taller than Perez, and Sauber managed to make the necessary changes to the car in time to get him out for the final 22 minutes of the second session. He ended up 18th, 0.8secs slower than Kobayashi.
The number of crashes 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.