Valtteri Bottas says Mercedes has "work to do" in order to get the best out of Pirelli's ultra-soft compound and beat Ferrari to pole position at the Canadian Grand Prix.

Lewis Hamilton led title rival Sebastian Vettel by just 0.198s in opening practice in Montreal, before Kimi Raikkonen put Ferrari on top in the afternoon session. Friday marked an encouraging return to form for Mercedes, after the German manufacturer registered its worst two-car finish since 2015 in Monaco. Bottas, who was third quickest in FP1 and fourth in FP2, believes the fight for pole in Canada will prove to be a close contest.

"I think today was better than in Monaco, I enjoyed it," he said. "It was tricky at times, especially with the ultra-soft tyre -- we have work to do on it. The car is feeling nice on the soft tyre, but once we put the ultra-soft on we didn't quite gain as much as we thought we would, so I think that is the main thing to keep in mind for qualifying. It's always fun! Of course it would be fun to be the quickest but we definitively still have work to do with the ultra-soft compound.

"I think it's going to be very close again and I'm already looking forward to the qualifying. I think it's going to be again about the last hundredths of a second and getting a good lap together so it should be fun. But we need to make sure that we now get more out of the ultra-soft tyre, much more than we did today. Then we can fight for the pole."

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Mercedes' performances have been inconsistent throughout 2017, particularly when it has been unable to get the ultra-soft tyre within the optimum temperature window to extract maximum performance -- something Ferrari has been able to do on a much more consistent basis. Its tyre-related issues were highlighted in Russia on Hamilton's side of the garage, while both cars struggled with tyre temperatures throughout the weekend in Monaco.

When asked if he understands why Mercedes requires specific temperature windows to get the ultra-soft tyre working, Bottas replied: "I wish! It's not ever easy to get all four tyres in the optimal window, getting them to work perfectly in every condition at every track. But we are trying hard and I think we got the softs to work really well today but with the ultra we have work to do. I think we know the direction we need to go to but we have plenty of time before qualifying so I think we will be competitive."

With the track surface particularly dirty and dusty due to a lack of regular track activity throughout the year, Bottas was among a number of drivers to spin on Friday. The Finn expects track conditions to improve dramatically ahead of qualifying on Saturday.

"It's always the same thing here," he explained. "On Friday's it's tricky and you see people making mistakes and I also spun once in Turn 1. With the tarmac type here and the snaps you have, it's really sudden so it's quite difficult to catch at times. It will be a lot better tomorrow when there is more rubber in the track and more grip."