Piastri, whose championship hopes began to crumble last year when he crashed on the tight road circuit with which he's had a "love-hate relationship", this time locked up with third place in his grasp with 10 laps left on Saturday and slid straight into the run-off.
By the time he got back on track, he was 15th, his hopes of any points gone, and he ended up 14th, with McLaren suffering double woe after Lando Norris had earlier crashed out, maddened at being the innocent victim in a multiple crash caused by Franco Colapinto.
Up ahead, Russell, who led from start to finish and survived the pressure of two restarts under the safety car, held off Max Verstappen in a dramatic finish to win his first race for three months and cut into Mercedes' teammate Kimi Antonelli's championship lead.
Russell managed to defend from Verstappen, who charged from eighth through to second, despite his Mercedes losing power in the denouement as Valtteri Bottas's last-lap crash in his Cadillac forced both the leaders to slow as the finish line approached.
At the end, Russell had less than two-tenths of a second to spare over his old rival, a margin rather smaller than he'd won his pole by in qualifying.
"I lost the boost, I had no power. Thought he was going to get us," a relieved Russell said.
Verstappen's Red Bull teammate Isack Hadjar was third in a superb return from injury.
Antonelli, who'd started 16th after a qualifying crash, ended up battling his way through to fifth, limiting the damage to his title lead as he's still 66 points ahead.
But Piastri couldn't hide his disappointment at the miserable conclusion to what had been a weekend when he'd seemingly rekindled his 'love' for the track where he'd won in 2024.
On the soft tyres, he'd whipped past Ferrari's Charles Leclerc at the start to grab second place and then held off the challenge of the Red Bulls impressively.
But after Alex Albon went into the wall on lap 31 and Piastri found himself on the mediums after everyone pitted under the yellow flag, he was picked off immediately by Verstappen on the restart.
Then, with the race having to be restarted again once Colapinto caused the crash which took out both his Alpine teammate Pierre Gasly and Norris, Piastri bemoaned his slide-off error while under pressure from Hadjar.
The mistake didn't impress outspoken 1997 world champion Jacques Villeneuve, who told Sky Sports: "It was a rookie mistake. He got caught out, looking in his mirrors, and just completely missed."
Piastri's view was, unsurprisingly, different. "I didn't think I braked super-late or super-hard, but just instantly locked both fronts," said the Victorian. "It certainly took me by surprise,
"Just a disappointing end to the day. The first half of the race was very strong, so it's a shame not to have anything to show for it."
Norris had been left dismayed by his race ending because Colapinto locked up while ploughing recklessly down the inside after the safety-car restart, an error which the Argentine apologised for profusely.
Norris, though, muttered darkly: "Just some drivers that shouldn't be in Formula One to be honest."
He felt the Argentine should get a one-race ban because "that's not driving that deserves to be in Formula One", but both Verstappen and Russell felt that a suspension would be too harsh.
