Lewis Hamilton led team-mate Valtteri Bottas as Mercedes made an impressive start to the Canadian GP with their upgraded engine in Practice One.
But true early comparisons between F1's Big Three will have to wait until the evening's second practice session as, unlike Mercedes, neither Ferrari nor Red Bull ran the fastest tyres available.
Hamilton, a six-time Montreal winner, posted a best lap of 1:12.767 which was 0.147s quicker than Bottas, who had led the early stages.
But there was late concern for Mercedes as Bottas pitted in the closing minutes with low fuel pressure. F1's world champions are investigating the problem ahead of Practice Two, which begins at 7pm on Sky F1.
Ferrari's Charles Leclerc and Red Bull's Max Verstappen were similarly-matched in third and fourth respectively, nine tenths off the pace, after running only the medium tyres.
Sebastian Vettel spun at the chicane and was fifth in the second Ferrari.
Crashes, spins and...groundhogs
Although the opening session of the weekend was slow to get going, a particularly dusty Circuit Gilles Villeneuve provoked plenty of incident and action as P1 progressed.
Alfa Romeo's Antonio Giovinazzi crashed out of the session after spinning into the barriers at Turn Nine, while a number of other drivers went off track - including Pierre Gasly, who cut across the grass at the first chicane.
And, in what has become an annual occurrence on the man-made Île Notre-Dame island that houses Canada's F1 track, one of the local groundhogs made an early appearance on circuit - with Nicholas Latifi forced to take avoiding action on his practice debut for Williams.
Raikkonen heads midfield
While one Alfa Romeo span out, the other finished a promising sixth fastest as Kimi Raikkonen edged out McLaren's Carlos Sainz and Renault's Daniel Ricciardo.
In fact, pointing to a competitive weekend ahead in F1's midfield, only Toro Rosso and current backmarkers Williams failed to get one of their cars in the top 10 during the opening session.
Mercedes customers Racing Point and Williams also ran the German manufacturer's latest engine.
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