The final hour of practice before qualifying began in cold and wet conditions after heavy rain, which had hit Montreal regularly over the past few days, arrived early enough on Saturday to disrupt proceedings.
At the start of the session on a soggy Gilles Villeneuve circuit, the four cars of Haas and Alfa Romeo cautiously set off for exploratory laps in extreme rain, soon joined by Ferrari duo Charles Leclerc and Carlos Sainz.
Haas’ Kevin Magnussen and Mick Schumacher put in good laps straight away in 1m40s as the puddles made the Quebec street circuit particularly treacherous, Magnussen settling on a 1m39.148s before heading to the garage.
While few other drivers were in a rush to get out, several recognized the need to get a feel for the conditions knowing that qualifying could take place under similar circumstances later.
After lining up the complex Turn 1/2, Aston Martin’s Sebastian Vettel found more grip to take the lead with a 1m39.041s then 1m38.294s after 15 minutes, which sparked a wave of improvements, with Alfa’s Valtteri Bottas who then clocked a 1m .37.909s.
Ferrari’s Carlos Sainz drastically lowered the benchmark before halfway with times of 1m36.831s and 1m35.858s amid countless off-track excursions from his colleagues.
The Red Bull and Mercedes cars joined the fray after halfway through the race – Mercedes without the double stay on the ground after whispers of rival protests – although the Red Bulls initially did not set a lap time.
Instead, it was Vettel who continued to rack up the miles and the four-time world champion also came out on top with a 1m35.821s with 20 minutes to go.
The German began to trade fastest times with Alpine’s Alonso in what felt like an echo of an earlier era, with Alonso’s 1m33.836s giving him the lead in the final 10 minutes as the two riders passed to intermediaries.
Magnussen caused the most notable incident of the session by going straight into Turn 1 and then crossing the track, narrowly keeping his Haas out of the Tecpro barriers while forcing Russell into evasive action.
Minutes later, world champion Max Verstappen suffered a similar scare after retiring in Turn 2 on splits, with the Dutchman also escaping undamaged.
In the dying seconds, AlphaTauri’s Pierre Gasly took second split session leader Alonso and Vettel.
Esteban Ocon took fourth, followed by McLaren duo Daniel Ricciardo and Lando Norris. George Russell was the first of the extreme wet riders in seventh.
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, with Lance Stroll further back in 13th and Magnussen 14th.
Hamilton had another tough 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 taking his fourth Ferrari power unit of the season, clocked five laps without a lap in 20th.