With very light rain limiting lap time improvements in the second half of the hour, Mercedes were confined to third and fifth places for Nico Rosberg and Lewis Hamilton respectively, split by the second Ferrari of Kimi Raikkonen.
Daniel Ricciardo was sixth fastest in the second Red Bull, with the top ten completed by Toro Rosso’s Carlos Sainz, McLaren’s Fernando Alonso, Force India's Sergio Perez and Williams’ Felipe Massa.
At the start of the session, the track and ambient temperatures were nowhere near what had been predicted, at 18 and 14 degrees Celsius respectively. That set up an interesting situation, where at times the supersoft Pirelli tyres seemed easier to heat up than the ultrasofts.
For a long time the Red Bulls, on the red-marked rubber, held sway. At one stage Verstappen and Ricciardo were dead even, with laps of 1m 14.636s, before the Dutchman edged ahead with 1m 14.310s. That was sufficient - just - to keep Hamilton (1m 14.334s) and Raikkonen (1m 14.341s), on the ultrasofts, at bay by 0.024s and 0.031s respectively as the final half hour approached. But then Vettel pushed ahead with 1m 14.139s and then 1m 14.068s on ultrasofts, 0.242s better than the Spanish Grand Prix winner. There was only 0.522s covering the two Ferraris, two Red Bulls and two Mercedes.
As the pressure ramped up in the final half hour, Vettel cut down to 1m 13.919s - the first sub-1m 14s lap of the weekend. The German and Hamilton also messed up Turn 1, possibly due to some rain down there. Rosberg meanwhile lost a better lap when coming across Magnussen’s Renault in the final corner. After backing off and trying again exactly the same thing happened again, but this time he improved to 1m 14.316s to go third. Verstappen, meanwhile, improved to 1m 14.158s to stay second.
With just over 15 minutes left, all of the main runners were in the pits, with reports of moisture in Turns 1 and 10 (the hairpin).
Soon, however, seventh-fastest Sainz reported the track to be dry again, prompting a wholesale exodus from the pits.
The red flag came out at 10.47, however, when Magnussen’s Renault curiously got away from him as he was exiting the second chicane at relatively low speed, oversteered, and smacked hard into the outer wall. The Dane was okay, but the RS16 was a mess.
The decision was taken not to restart the session as the wall was also damaged, leaving Vettel ahead of Verstappen, Rosberg, Raikkonen, Hamilton, Ricciardo and Sainz, with only 0.736s between them.
Alonso was an encouraging eighth for McLaren with 1m 14.810s and Perez (1m 14.886s) and Massa (1m 14.890s) added variety as they filled out the top 10.
The session may have ended in anti-climax as we weren’t able to see everyone’s hand in a final pre-qualifying blast, but the official grid-forming session this afternoon promises to be exceptionally close - and could spring surprises.
WATCH: Magnussen falls prey to the Montreal walls
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