* Mercedes' run of four poles in a row ends
* Bottas third fastest for Williams (Updates with Raikkonen demoted to 10th, adds Massa quotes)
By Julian Linden
MONTREAL, June 8 (Reuters) - Sebastian Vettel snatched pole position at the Canadian Grand Prix for the third year in a row Saturday, pipping Lewis Hamilton in a frantic wet qualifying session to end Mercedes' run at the front of the grid.
The triple world champion kept his cool in the treacherous conditions to safely steer his Red Bull around the Montreal street circuit in a fastest time of one minute 25.425 seconds for his 39th career pole.
"It was very, very tricky because you never knew what the conditions were like," Vettel said.
"We're extremely happy with the result and looking forward to the race tomorrow. No matter what the conditions, I think we're in good shape."
In a repeat of last year's qualifying, the 25-year-old German was joined on the front row of the grid by Britain's 2008 world champion Hamilton, who was just 0.087 seconds behind.
Finnish rookie Valtteri Bottas was third for Williams, the highest grid placing of his short career and a major boost for a team suffering their worst start to a season with no points in six races so far.
Monaco winner Nico Rosberg, who had taken pole at each of the last three rounds of the championship, will line up fourth for Mercedes.
Red Bull's Australian Mark Webber and Ferrari's Fernando Alonso shared the third row while Lotus's Kimi Raikkonen, second in the championship standings behind Vettel, qualified ninth but was relegated to 10th after a pit-lane infringement.
Despite his run of poles in Canada, Vettel has yet to win a Formula One race in North America despite his domination elsewhere but gave himself another great opportunity.
"It was really tricky conditions and difficult to get the timing right but Sebastian did it just perfectly," Red Bull team boss Christian Horner said.
"Pole is the best place to be starting and having Mark in fifth is still a strong result on a track where you can overtake. The guys have done a great job today."
PUDDLES OF WATER
With light rain falling and puddles on parts of the track, none of the drivers were able to drive flat out, struggling just to stay on the slippery Circuit Gilles Villeneuve, one of the most demanding courses in Formula One because of its super-fast long straights and slow, tight corners.
Most drivers, including Vettel, ran off course at some point during the three-stage qualifying session, which was halted in the middle when Ferrari's Felipe Massa slammed sideways into a barrier.
The Brazilian, who also crashed heavily in Monaco practice and the race, qualified 16th.
"Physically, I'm fine, but within myself I'm very disappointed," Massa said. "Never before have I had three accidents in such close succession."
The day got off to an ominous start when the final practice session was reduced from 60 minutes to 30 after a car crashed into a fence during a support race, forcing maintenance staff to repair the barrer before the F1 cars could run.
Vettel had taken pole in the first two races this season, in Australia and Malaysia, but the Red Bulls had been unable to keep up with Mercedes in the four races since.
A three-time winner in Canada with his previous team McLaren, Hamilton had loomed as the man to beat for pole but the Englishman was unable to get enough heat in his tyres to generate the speed he needed.
"I was doing a pretty good final lap but I ran wide, so it was unfortunate but I'm still happy," said Hamilton.
McLaren failed to make the final phase of qualifying on Saturday with Jenson Button, a winner in Canada two years ago from last place after a re-start, qualifying 14th and missing the top-10 shootout.
"To be fair, I'm not too upset. It's pretty scary out there," he said. "I had my eyes closed for most of it. We struggled to get temperature into the tyres. It just didn't go our way."
While there was disappointment for McLaren and Ferrari, the struggling Williams team were in high spirits after Bottas weaved his way through the traffic to qualify third.
"This means a lot. It's way more up in the grid than we could have imagined," said the 23-year-old, who has been tipped as a future star and has retired double champion Mika Hakkinen as a mentor.
"The first six races have been so difficult, so this will be a nice boost for the team. We got everything just right today." (Reporting by Julian Linden; editing by Alan Baldwin)