Lewis Hamilton top in Canadian Grand Prix first practice
Mercedes driver Lewis Hamilton edged title rival Sebastian Vettel of Ferrari to set the pace in first practice at the Canadian Grand Prix.
Hamilton, trailing Vettel by 25 points in the championship, was 0.198seconds clear of the German with their team-mates Valtteri Bottas and Kimi Raikkonen next.
Fernando Alonso's return to Formula 1 after missing the Monaco Grand Prix ended with the Spaniard stopping on track.
The two-time champion suffered a lack of hydraulic pressure, McLaren said.
The team were unable to say what had caused the problem.
Pressure builds on Honda
Alonso was told by his engineer to stop on track. He pointed out that was he already at the hairpin, the last corner before the pit entry and it was easier to come back to the pits, but was ordered to stop.
He flung the impact-protection foam out of his cockpit as he climbed out and was given a standing ovation and cheers by the crowd in the grandstands.
"We are used to it," he told reporters on the way back to the pits.
The failure came as McLaren executive director Zak Brown again said that Honda had to up their game if the partnership was to have a future.
Two days after saying he had "serious concerns" about Honda, and a day after Alonso indicated he could leave McLaren at the end of the year if they were not winning by September, Brown told BBC Sport: "The plan right now is to have the Honda in the back of the car (in 2018) but some things need to happen between now and then for us to have the confidence we can be at the front of the field next year.
"We need to get competitive and show regular signs that we are getting competitive.
"Right now, we're not racing well, not finishing races and that can't happen any more.
"We are starting to work on the 2018 car so we need to make any decisions that impact 2018 by the summer break. Something needs to change. If you keep doing the same thing, you are going to get the same result. Maybe take some risks, do things they wouldn't normally do. You can't keep doing the same thing and expect things to change."
Asked if they had been discussing a customer engine supply with Mercedes, Brown said: "We have a plan B, a Plan C. We have some plans."
Mercedes back on form?
Back at the front, it was for Mercedes an encouraging start to a weekend when ideally they need to start clawing back some ground in the championship.
A poor weekend for Hamilton in Monaco allowed Vettel to extend a lead of nearly a a clear win by winning the race.
The silver and red cars appeared closely matched throughout the session on a tricky, slippery Circuit Gilles Villeneuve that provoked a number of spins through the first session.
The combination of a low-grip, low-abrasion track surface and asphalt that is in a park and rarely used for the rest of the year and therefore dusty and dirty makes for a treacherous experience for the drivers, with unyielding walls waiting for those who make mistakes in the wrong places.
Behind Mercedes and Ferrari, Force India impressed, with Sergio Perez and Esteban Ocon fifth and sixth quickest ahead of the Red Bull of Max Verstappen, who was separated from team-mate Daniel Ricciardo by Felipe Massa's Williams.
Massa's team-mate Lance Stroll was five places and 1.2secs behind on his first experience at his home race.
Alonso's team-mate Stoffel Vandoorne was 11th quickest, with the Spaniard's single lap putting him 16th.