Ferrari's Fernando Alonso sprung a surprise by setting the pace ahead of the title-contending Mercedes in first practice at the Canadian Grand Prix.
Alonso was 0.016 seconds ahead of Lewis Hamilton, who was 0.13secs ahead of team-mate Nico Rosberg.
Red Bull's Sebastian Vettel was fourth from Williams' Valtteri Bottas, Red Bull's Daniel Ricciardo and McLaren's Jenson Button and Kevin Magnussen.
Hamilton was on course to beat Alonso before running wide at the hairpin.
Mercedes are expected to be in a league of their own on the Circuit Gilles Villeneuve.
The track's long straights penalise the power deficit of the cars with the rival Renault and Ferrari engines, and should enable Mercedes customer teams Williams, McLaren and Force India to compete with Red Bull and Ferrari.
But Ferrari have a number of upgrades in Montreal which they hope will boost their competitiveness - among them increased power from the engine and revised aerodynamic parts, including a new floor.
And Alonso was able to mix it with the Mercedes throughout the session.
He was third fastest after the first runs, and then moved into second and then first place as the session progressed.
His team-mate Kimi Raikkonen, running his car in a different configuration as Ferrari assessed their new parts, was 1.34secs behind his team-mate in ninth.
BBC F1 analyst Allan McNish, watching at the final chicane, said: "There is a big difference between the behaviour of the two Ferraris on the track.
"Raikkonen doesn't seem to be very confident on the brakes. The car is doing two or three different things as if it's on and off into the apex. Alonso is a little more composed and more confident with the car.
"But the two cars are running in different specifications. There is a new floor and Alonso is running it in the first session and then Raikkonen will back-to-back it in the second."
Hamilton's incident was one of a number of off-track moments by several drivers on the demanding track.
A quasi-street circuit built on the man-made Ile Notre Dame in the middle of the St Lawrence Seaway close to downtown Montreal, the Circuit Gilles Villeneuve is characterised by long straights, hard braking and walls in close proximity to the track.
A number of drivers had near-misses as they explored the limits on the dirty track surface, but Marussia's Jules Bianchi paid the price for a mistake by hitting the wall and breaking his rear suspension at the exit of Turn Four.