Piastri pinched the lead with a gutsy overtake into the first turn on lap 20, afterwards putting on a masterclass of defensive driving to hold back Leclerc for 29 laps before the Ferrari driver’s tyres finally expired from the strain of the pursuit.
Leclerc’s dead tyres looked sure to lose him even more places, but Sergio Pérez and the fast-finishing Carlos Sainz crashed out of the battle to for the right to pick him off on the penultimate lap.
Piastri had two opportunities to vie for victory after Leclerc aced his start from pole to lead into the first turn. With the race expected to be run with just a single pit stop, the Australian knew that the clean air of the lead would be crucial to getting the tyres to the end and so pushed Leclerc hard in the opening laps looking for a gap in his defences.
It bit the McLaren driver hard, however, with his medium-compound tyres overheating quickly, dropping him almost six second off the lead as the pit stop window opened.
Pérez, who was running a close third in comfortably his best weekend performance since April, sought to capitalise by pitting early, on lap 13, but Red Bull Racing’s undercut was foiled by Lando Norris. The Briton started 15th on the grid after a disastrous qualifying, but Pérez’s early stop dropped him just behind the sister McLaren, which duly did its best to hold him up through the twisty middle sector.
It was enough to ensure Piastri could pit and rejoin with net second place intact.
Leclerc followed one lap later, but a slow lap into pit lane and a slower one rejoining the race sliced what had been a healthy buffer to practically nothing.
Piastri, with his hard tyres benefitting from an extra lap of temperature in them, saw his opportunity to pounce.
He picked his moment on lap 20 with a great exit from the last braking zone giving him strong run down the 2.2-kilometre front straight into the first turn.
Leclerc watched the Australian in his mirrors and was relieved to see him too far back to make a move when he hit the brakes - or so he thought.
With a late-braking move that would have made compatriot Daniel Ricciardo proud, Piastri dived ambitiously down the unguarded apex and pulled up his McLaren just in time to make the corner, picking Leclerc’s pocket and changing the complexion of the race.
Leclerc had been unfazed by the move, assuming he’d have an opportunity to fight back later in the race, but he found his parries easily rebuffed as the laps wore on. Piastri was nailing his exit from the last braking zone to make the most of McLaren’s small top-speed advantage, keeping the door firmly closed.
The pressure was immense from Baku specialist Leclerc, but Piastri never cracked. Cool and collected in the face of an assault from a driver with almost four times as many starts in a car that was more than 0.3 seconds quicker than qualifying, he broke the challenge in the final laps to claim his second career victory.
“I think that ranks as probably the best win of my career,” he said. "Trying to soak up that pressure for so long in that race that was incredibly tough.”
Leclerc was bitterly disappointed to have failed to convert pole to victory for the fourth Azerbaijan Grand Prix in succession, lamenting that his car hadn’t been set up to be more competitive in race trim.
“We had a bit more downforce, which made us quite quick in the castle section; however, in all the straights they were flying,” he said. “That’s probably where I lost the race. I misjudged that.”
Pérez had seemed sure to finish at least third before his tangle with Sainz wiped both drivers out of the race.
Sainz had slipped past the Mexican at the first turn of the penultimate lap but missed the apex at the second corner attempting to size up his teammate.
Pérez slipped down his inside to run him side by side down to turn 3, but they exited the corner with intersecting lines. Neither driver was willing to deviate, putting both in the wall in what the stewards judged a racing incident.
George Russell drove through the debris field to collect an unlikely third place on a lukewarm weekend for Mercedes, with the Briton’s race looking uncompetitive until the final 20 laps, when his hard tyres suddenly came alive.
“We’ve got to be realistic. We should have finished fifth today; that was the true result,” he said. “I don’t want to get carried away with ourselves with this podium today. We’ve got a lot of work to do.”
Lando Norris completed a disciplined recovery drive from 15th on the grid to overtake title rival Max Verstappen with two laps to go, compounding the Dutchman’s poor weekend by reducing the championship gap to 59 points. Verstappen put his disappointing race down to a set-up gamble made before qualifying that left him lacking confidence in the car.
Fernando Alonso was a lonely sixth ahead of superb Williams duo Alex Albon and Franco Colapinto, who combined to score 10 points to move the team up to eighth in the constructors standings ahead of Alpine. Colapinto, in just his second grand prix and first visit to Baku, was particularly impressive, running a 41-lap stint on a single set of tyres to finish the race.
Lewis Hamilton recovered from a pit lane start for an engine change to finish ninth ahead of an excellent Oliver Bearman, who took the suspended Kevin Magnussen’s car to 10th, one place ahead of teammate Nico Hülkenberg, in just his second F1 start.