Lewis Hamilton said he expected a "tough" race in Canada trying to keep in touch with Red Bull's Sebastian Vettel and Ferrari's Fernando Alonso.
The McLaren driver has qualified second in Montreal, behind Vettel and ahead of Alonso but said he expected their cars to be faster in the race.
"It's going to be really tough. The Ferrari has the best long-run pace and the Red Bull probably second," he said.
"I think it's Fernando we have to watch out for mostly."
Hamilton heads into the race 13 points behind championship leader Alonso, who is three points ahead of the Red Bull duo of Vettel and Mark Webber, who starts fourth.
His main concern will be tyre life - with which he said that Red Bull and Ferrari had proved to have an advantage in the last race in Monaco.
McLaren have also made changes to their pit-stop procedures in an attempt to prevent the problems that have afflicted them in recent races.
"I'll do everything I can in the race to look after the tyres," Hamilton said, "but I did that in the last race and they went off. It may be different here. I don't know what to expect.
"I'm going to remain in a positive mind, and I know the guys have been working really hard on the pit stops, so I'm sure it's going to be a better race tomorrow."
He added: "Ferrari have incredible degradation. They didn't have a lot of degradation in the last race and they were phenomenally quick and the Red Bull was very quick as well.
"They were able to do something like 40 laps on an option tyre. We're not able to do that yet, I think we need a little bit more downforce for that, but we'll give it our best and hope we can remain in the battle.
"If we go forward, I'm going to be very happy."
The Englishman said he was pleased and surprised to manage to qualify on the front row.
"I thought I would be behind a Ferrari and maybe a Mercedes," Hamilton said.
"I was really grateful we were on the front row. Race pace, it's much hotter, but we'll do what we can."
Ferrari have introduced a major re-design to the rear of their car in Canada, featuring revised rear bodywork and a new exhaust position aimed at enhancing aerodynamic efficiency.
The result was that the team had their best collective qualifying performance of the year, with Alonso third and team-mate Felipe Massa sixth on the grid.
"The car did improve this weekend," the Spaniard said. "We made some good progress with the new parts and we felt much more confident and competitive in free practice.
"Qualifying is the time to confirm that and we did so with Felipe and me into Q3 and top six for Ferrari.
"It's very good news for us in terms of championship possibilities because until now we were leading the championship, but with some up and downs for everybody. Now with a car that can be competitive and fight with the others we feel much more confident.
"It is maybe a weekend to gain some confidence because sometimes you bring some updates to the car and some of them they work and some not.
"But it's like the fourth or fifth consecutive time we have brought something and it has been positive so it is a good trend that we need to keep."