The Monaco race winner trailed the defending champion by some four tenths in the morning before finishing behind Hamilton, Vettel, and Raikkonen in the rain-affected afternoon running.
Ferrari is bringing an upgraded power unit to Montreal after spending their first three development tokens in an effort to reel in frontrunners Mercedes.
“We ran low fuel with the soft tyre and it’s a bit difficult to warm up the tyre because it’s quite cold temperatures,” said Rosberg. “But balance and everything was OK. Ferrari looks pretty close so we need to look into that.
“I haven’t really spoken to the engineers yet so I don’t know what their real pace is relative to us, because we always know their fuel amount, how much they were turning up their engine – we can see all that and it means much more than the actual times eventually. But I’m not sure what the end result on that is yet.”
Rosberg also explained why both he and Hamilton surprisingly ventured out on track with inters, as rain was pouring down on Circuit Gilles Villeneuve during the second free practice session.
“We just went out to try some starts. We didn’t want to drive fast or anything, we just wanted to try starts.”
While the German was able to complete one lap and return safely to the pits, the championship leader ended up aquaplaning off track at Casino hairpin and crashed into the tyre wall.
Click here for technical analysis of the braking challenge at the Canadian Grand Prix