The Australian driver could only finish 11th as Mercedes young gun Kimi Antonelli claimed a fourth straight grand prix victory in Montreal on Sunday.
A day after clashing with George Russell in a heated sprint race, Antonelli fought a tense duel with his Mercedes teammate for 30 laps before Russell dropped out with engine failure.
Antonelli won by 11 seconds from Ferrari veteran Lewis Hamilton, with Red Bull's Max Verstappen third.
The 19-year-old Italian opened a commanding 43-point championship lead over Russell through five of 22 races.
But it was a grim day for McLaren, who came away with zero points from the race, hampered following a decision to start on intermediate tyres, expecting rain, while others were on slicks.Â
Piastri dropped to fifth at the start and both cars pitted early to change tyres before Lando Norris retired with a gearbox issue on lap 40.
The race got worse for a straggling Piastri when he made contact with Alex Albon's Williams in a hairpin corner, damaging his car and incurring a 10-second penalty.
"It was raining and between the anthem and getting in the car, it was pretty wet on the ground," said Piastri post-race.
"Unfortunately for us, it stopped raining. If it rained a little bit more we would have looked like heroes. It didn't, so we looked like idiots."
The Australian is now a distant 83 points behind Antonelli in sixth place (48 points) in the drivers' standings with Norris 10 points ahead in fifth spot.
"That was a day," said Piastri. "Not great from start to finish so plenty to learn.
"I feel like in some areas we made a bit of progress. We were able to get closer to Mercedes (in car speed)."
McLaren team principal Andrea Stella explained the call made on tyres.
"The decison to start on intermediate tyres was not rewarded. You have to decide what tyres to fit five minutes before the start and at the time the track was greasy and it was still raining," Stella said.
"But as soon as we fitted the tyres the rain stopped and the track dried out pretty rapidly and the fact there were two extra formation laps took away the full advantage of this decision.
"This is racing. It is in the difficult days that you have to demonstrate the character and you have to learn the qualities that make you a champion and today is one of those days."
- with The AP
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