MONTREAL (AFP) - Nico Rosberg boosted his hopes of claiming a third straight win when he topped the times for Mercedes in Saturday's unpredictable and dramatic final free practice session ahead of Sunday's Canadian Grand Prix.
The 29-year-old German clocked a best time of one minute and 15.660, the quickest lap of the weekend to date, to soar to the top of the lists, while his team-mate and defending world champion Lewis Hamilton wound up in 20th and last place.
Hamilton was unable to deliver a single lap with any consistency and rhythm on a morning of difficult, changeable conditions interrupted twice by red flag stoppages.
Rosberg, the winner in Spain and Monaco, was quickest by 0.573 seconds, ahead of Kimi Raikkonen of Ferrari and Romain Grosjean of Lotus with Valtteri Bottas fourth for Williams.
Sergio Perez was fifth for Force India ahead of Daniil Kvyat of Red Bull, Felipe Massa in the second Williams, four-time champion Sebastian Vettel in the second Ferrari, Spanish rookie Carlos Sainz of Toro Rosso and Venezuelan Pastor Maldonado in the second Lotus.
Hamilton, who survived an off-track excursion, was five seconds off the pace at the back of the field, but his problems were nothing as to those of the luckless McLaren team.
Having changed the engine on two-time champion Fernando Alonso's car to enable him to run for the final 15 minutes, they saw the 2009 champion Jenson Button slow to a halt with an engine failure.
That spelt another engine change for the team and signalled he was out of qualifying later on Saturday, the team confirmed.
Alonso's new engine was his permitted maximum fourth of the season while Dutch teenager Max Verstappen, of Toro Rosso, also went for a new engine - his fifth of the year - and faces a 10-place grid penalty.
He already has a five place penalty for causing a collision with Grosjean at the Monaco Grand Prix.
Button's engine failure was the cause of the second red flag in the closing minutes. Felipe Nasr had caused the first when he lost control of his Sauber and smashed into a wall on the back straight towards the final corner as he weaved to warm his tyres.
The Brazilian driver climbed out and walked away to be taken swiftly to the circuit medical centre for a check-up. The session was red-flagged and delayed, finally restarting with 12 minutes remaining.
"I had a big shunt," he said. "The tyres were way too cold."