MONTREAL (AP) -- Whether it's practice, qualifying or the races themselves, Mercedes teammates Lewis Hamilton and Nico Rosberg have been dominating Formula One.
Winners of all six races so far this season, the two drivers posted the fastest laps on the first day of practice Friday at the Canadian Grand Prix. Hamilton turned in the top time of 1 minute, 16.118 seconds in the afternoon practice -- a speed of 128.16 mph (206.253) -- on the 2.71-mile (4.36-kilometer) Circuit Gilles Villeneuve, and Rosberg was just 0.175 seconds slower.
"Nico looked very quick today as well so I have my work cut out for sure," said Hamilton, who won four races in a row before finishing second to Rosberg at Monaco last month. "The Ferraris looked quite close today and the rest of the field seemed closer in general but hopefully not too close."
Rosberg leads the Formula One championship with 122 points, thanks to two victories and four second-place finishes. Hamilton, the 2008 champion, is in second place for the season with 118 points from four victories and a No. 2 finish; he lasted just two laps before a failed cylinder knocked him out of the season-opening Australian Grand Prix.
Hamilton won at the track for McLaren in 2007, 2010 and 2012.
Ferrari's Fernando Alonso is in third place with just 61 points. He had the fastest lap in the morning practice Friday with a time of 1:17.238.
"It's going to be a tough challenge" to catch Mercedes, Ferrari engineering director Pat Fry said. "But we just need to keep on. There's quite a gap to close, let's face it, but we just need to do our best and keep developing."
Fry said Ferrari's garage was busy Friday after problems with Kimi Raikkonen's power unit limited him to just 15 laps in the morning. But he said both drivers got enough time on the track to provide data that the team can evaluate before practice resumes on Saturday morning.
"It's too early to say whether it's all working or not," Fry said. "Some things are looking promising; some we need to look into in more detail. ... But we need to keep developing the car as quickly as we can."
Although it was surprising to see the dominant Mercedes knocked from the top spot in practice, Rosberg said he was experimenting with the brakes in the morning before making a change for the afternoon.
"It was much better for the second session, so I'm quite happy now with that," he said. "But I'm still a tenth off the quickest time so I'll work hard with our engineers tonight to find that extra pace."
Four-time defending Formula One champion Sebastian Vettel, the race winner last year, posted the third-fastest lap of the afternoon, with Ferrari's Raikkonen and Alonso fourth and fifth. Williams teammates Felipe Massa and Valtteri Bottas were sixth and seventh.
Light rain fell in the late morning but did not disrupt practice. Teams ran more laps in the morning because of the threat of more rain, but the second session went on without interruption.
Another practice session on Saturday morning will be followed by qualifying in the afternoon.
"It's very hard to overtake here so pole position is important," Hamilton said.
Mercedes executive director Paddy Lowe said the focus on Friday was doing their homework on tire performance and fuel consumption.
By the afternoon, they seemed to have figured it out.
"It's not easy," Lowe said. "I know we have a fantastic record so far this year, but we've got some competitors out there who will grab everything we leave behind. We've got to make sure we don't."
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