Kimi Antonelli led George Russell to a Mercedes one-two in the sole practice session, which was hampered by three red flags, at the Canadian Grand Prix.
Antonelli, who is leading the drivers' championship, was 0.142 seconds quicker than Russell, setting a time of 1:13.402 on the soft tyre.
Lewis Hamilton was over seven tenths off the pace in third, ahead of Ferrari teammate Charles Leclerc at the Circuit Gilles Villeneuve.
Max Verstappen rounded out the top five in the Red Bull ahead of sprint qualifying later.
Mercedes unveiled its first major upgrade package of the season as the grid's dominant team in 2026, having won each of the four races so far, bid to stretch its advantage to the rest of the field.
Antonelli, 19, has turned the tables on his experienced teammate Russell by winning the last three races and arrives in Montreal with a 20-point advantage.
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Russell enjoys this circuit, having won here last season and claimed pole in the last two years, raising the pressure for him to deliver this weekend.
But it was the Italian who was again top of the standings in the first running of the weekend.
Russell suffered a spin late in the session, narrowly keeping it out of the barriers, as his younger teammate landed the first blow.
McLaren has also brought the second half of its upgrades package to Montreal, introducing a new front wing as part of a raft of changes aimed at dragging Lando Norris and Oscar Piastri back into the championship picture.
Norris was only sixth, an eye-watering 1.397 seconds behind Antonelli.
The first red flag of the session was caused by Racing Bulls' Liam Lawson, who had to stop on track as he was stuck in gear. Not long after, an incident involving a groundhog and Alex Albon's Williams caused him to crash while he was exiting Turn 7.
The session was extended by 15 minutes and it was Esteban Ocon who collided with the wall in his Haas, bringing out the third red flag.
PA contributed to this report.
