Fernando Alonso claimed the opening P1 for the Canadian Grand Prix weekend having beaten Lewis Hamilton by 0.016s.
McLaren rookie Kevin Magnussen was the first out on a dirty Circuit Gilles Villeneuve, clocking a 1:23.2. And although he lowered the benchmark with each consecutive lap, he could do nothing the stop the Mercedes drivers from gaining control.
Lewis Hamilton, despite an early scare that turned out to be a radio issue, hit the front with a 1:18.997, easily half a second up on second-placed Felipe Massa.
It soon became a Mercedes 1-2 as Nico Rosberg closed up on his team-mate, the German 0.03s down.
The duo went even quicker, a 1:18.088 for Hamilton and a 1:18.138 for Rosberg, while Fernando Alonso slotted his Ferrari into third place, half a second slower on the P1 time.
With dark clouds closing in on the dirty track, drivers struggled to get heat into their tyres with Caterham reserve driver Alexander Rossi told his tyre temperature is at the "bottom of the working range."
That explained his lock ups as well as issues other drivers such as Pastor Maldonado were experiencing.
Back at the front and out for a second run, Rosberg went quickest ahead of Hamilton only for the Brit to overhaul him.
It was all change, though, as Alonso posted a 1:17.238, edging Hamilton by 0.016s.
When the Mercedes drivers came out for their final run both were fueled up meaning Alonso finished quickest of all ahead of the Brackley racers.
Sebastian Vettel was fourth quickest ahead of Valtteri Bottas and Daniel Ricciardo.
There were problems for Monaco points-scorer Jules Bianchi as he hit the wall, not the Wall of Champions, just the wall. That has left him with a "maybe a track rod failure" he said as he crawled back to the pits with a seriously vibrating Marussia.
Meanwhile, an "ERS pump failure" meant Massa managed just seven laps as his team worked on fixing the problem while a "control unit" issue curtailed Esteban Gutierrez's morning.
1 Fernando Alonso Ferrari 1:17.238 21 laps
2 Lewis Hamilton Mercedes 1:17.254 +0.016 25 laps
3 Nico Rosberg Mercedes 1:17.384 +0.146 32 laps
4 Sebastian Vettel Red Bull 1:18.131 +0.893 28 laps
5 Valtteri Bottas Williams 1:18.361 +1.123 20 laps
6 Daniel Ricciardo Red Bull 1:18.435 +1.197 26 laps
7 Jenson Button McLaren 1:18.446 +1.208 33 laps
8 Kevin Magnussen McLaren 1:18.514 +1.276 31 laps
9 Kimi Raikkonen Ferrari 1:18.578 +1.340 15 laps
10 Jean-Eric Vergne Toro Rosso 1:18.643 +1.405 14 laps
11 Nico Hulkenberg Force India 1:18.733 +1.495 30 laps
12 Sergio Perez Force India 1:18.959 +1.721 22 laps
13 Adrian Sutil Sauber 1:19.108 +1.870 24 laps
14 Romain Grosjean Lotus 1:19.142 +1.904 32 laps
15 Daniil Kvyat Toro Rosso 1:19.177 +1.939 21 laps
16 Pastor Maldonado Lotus 1:19.340 +2.102 37 laps
17 Felipe Massa Williams 1:19.575 +2.337 7 laps
18 Esteban Gutierrez Sauber 1:19.804 +2.566 15 laps
19 Jules Bianchi Marussia 1:20.200 +2.962 15 laps
20 Max Chilton Marussia 1:20.844 +3.606 26 laps
21 Marcus Ericsson Caterham 1:21.404 +4.166 33 laps
22 Alexander Rossi Caterham 1:21.757 +4.519 27 laps
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