The final hour of practice before qualifying began in cold and wet conditions after heavy rains, which had hit Montreal regularly in recent days, arrived early enough on Saturday to disrupt proceedings.
Early in the session at a sodden Circuit Gilles Villeneuve, the four cars of Haas and Alfa Romeo cautiously set out for reconnaissance laps in extreme rain, soon followed by Ferrari pairing Charles Leclerc and Carlos Sainz.
Kevin Magnussen and Mick Schumacher from Haas immediately drove good laps in the 1:40 minutes, as puddles made the street circuit in Quebec particularly treacherous. Magnussen settled on 1m39.148s before pitting.
While few other drivers rushed to get out, some recognized the need to get a feel for the conditions, knowing that qualifying could take place later in similar circumstances.
After the straight line at Turn 1/2, Aston Martin’s Sebastian Vettel found more grip to take the top spot with 1m39.041s and then 1m38.294s after 15 minutes, sparking a raft of improvements, with Alfa’s Valtteri Bottas then a 1 meter mark drove .37.909s.
Ferrari’s Carlos Sainz dramatically lowered the benchmark before half-time with times of 1m36.831s and 1m35.858s amid countless off-track excursions from his peers.
The Red Bull and Mercedes cars joined the fray after half-time – Mercedes without a double remained on the ground after murmurs of protest from the competitors – although the Red Bulls initially did not set any lap times.
Instead, it was Vettel who continued to rack up the kilometers and the four-time world champion also took the lead with 20 minutes to go with a 1m35.821s.
The German began trading fastest times with Alpine’s Alonso in what felt like an echo from a bygone era, with Alonso’s 1m33.836s giving him the lead in the last 10 minutes as both riders switched to intermediates.
Magnussen caused the most notable incident of the session by going straight at Turn 1 and then crossing the track, keeping his Haas just out of the Tecpro barriers while forcing Russell to take action.
Minutes later World Champion Max Verstappen suffered a similar shock when he spun onto Intermediates at Turn 2 and the Dutchman also escaped unscathed.
In the final seconds, AlphaTauri’s Pierre Gasly overtook second-placed session leaders Alonso and Vettel.
Esteban Ocon was fourth, followed by McLaren duo Daniel Ricciardo and Lando Norris. George Russell was the first of the extreme wet runners in seventh place.
The Red Bull duo Sergio Perez and Verstappen followed in eighth and ninth place ahead of Sainz.
The Alfa Romeos were next with Bottas and Zhou Guanyu in 11th and 12th respectively, Lance Stroll further back in 13th and Magnussen in 14th.
Hamilton had another difficult session in 15th, followed by Alex Albon’s Williams and Tsunoda’s AlphaTauri.
Schumacher and Nicholas Latifi’s second Williams languished in 18th and 19th.
Leclerc, who will start from the back of the grid after driving his fourth Ferrari engine of the season, completed five laps without a lap time in 20th place.