MONTREAL, Canada -- Nico Rosberg thinks Mercedes' rivals did not show their true pace during Friday practice for the Canadian Grand Prix.

Rosberg finished the session third fastest, 0.269s behind Ferrari's Sebastian Vettel and 0.418s faster than the quickest Red Bull of Max Verstappen. But Rosberg thinks the true picture is clouded by fuel loads and Red Bull opting to run a low power setting on its recently upgraded engine.

"The long run is looking good, our car is looking good but Ferrari is looking close today," he said. "It is always so difficult on Friday you never know what fuel levels they are running, how much engine power they are using. At Red Bull they had their engine turned down completely which didn't show anything so let's see."

Rosberg was 0.526s off team-mate Lewis Hamilton and said his struggles stemmed from the cool temperatures in Canada.

"Generally I feel good in the car. But getting the tyres working is not so straightforward here, so that needs some further analysis tonight. I lost most of my time in the first sector - possibly because of tyre temperatures. So, that part of the lap is the main area to work on and find some improvement for qualifying.

"The long runs have looked good so far, though. It's been great to see so many spectators out there already today and I'm sure there will be an awesome atmosphere over the weekend."