SPA-FRANCORCHAMPS, Belgium - Oscar Piastri passed McLaren teammate and title rival Lando Norris with a bold early move to win the rain-delayed Belgian Grand Prix and extend his Formula One lead to 16 points on Sunday.

Charles Leclerc was a distant third for Ferrari as reigning champions McLaren celebrated their sixth one-two finish in 13 races and the third in a row.

The race at Spa-Francorchamps was red-flagged after an initial formation lap and delayed by an hour and 20 minutes due to the weather, with standing water and heavy spray affecting visibility.

Piastri was in no mood for hanging around when the racing got going with a rolling start after four laps behind the safety car to check conditions.

The Australian slipstreamed Norris through the daunting Eau Rouge and then scythed past down the Kemmel straight into Les Combes in a move of total commitment in the treacherous conditions.

"I knew lap one would be my best chance of winning the race. I got a good exit out of Turn One, lifted as little as I dared out of Eau Rouge," he said.

"The rest of the race we managed really well. I struggled at the end. Maybe the mediums were not the best for the last five or six laps. We had it mostly under control."

The win was his sixth of the season, making the 24-year-old the first Australian - on a list that includes past world champions Jack Brabham and Alan Jones - to triumph so many times in a single campaign.

Norris had a slight battery issue, the Briton asking over the radio why he had "no pack" before his race engineer assured him it was coming back, but he was not looking for any excuses afterwards.

"Oscar just did a good job. Nothing more to say. Committed a bit more through Eau Rouge, and had the slipstream and got the run," he said.

"So nothing to complain of. He did a better job in the beginning, and that was it. Nothing more I could do after that point. I would love to be up top, but Oscar deserved it today."

Two-horse race

Piastri now has 266 points to Norris' 250. Red Bull's reigning champion Max Verstappen is third but 81 points off the lead and the championship more than ever a two-horse race with Hungary up next weekend before the August break.

McLaren lead the constructors' standings with 516 points to Ferrari's 248 while Mercedes fell further behind their Italian rivals on 220.

Piastri pitted on lap 12 of 44 to switch from intermediates to medium tyres on a drying track and Norris followed a lap later, but opting for the hards and rejoining nine seconds behind.

The Briton might have hoped Piastri would have to pit again but the Australian made the tyres last to the chequered flag on a one-stop strategy.

Piastri crossed the line 3.415 seconds clear of Norris, who had been chasing a third win in a row and managed to reduce the gap in the final laps before late mistakes left the ever-calm Australian under no pressure.

Saturday sprint winner Verstappen finished fourth in his team's first grand prix since the dismissal of team boss Christian Horner , with George Russell fifth for Mercedes.

READ MORE: Norris wins chaotic F1 British Grand Prix after Piastri penalty

Williams' Alex Albon held off Ferrari's seven-times world champion Lewis Hamilton - last year's winner with Mercedes - to secure sixth.

Hamilton had been one of four drivers due to start from the pit lane but given a big boost by the switch to a rolling getaway and a fresh engine installed overnight.

The Briton was also the first to make the decision to switch to slicks and pit, gaining six places.

Liam Lawson was eighth for Racing Bulls with Gabriel Bortoleto ninth for Sauber and Pierre Gasly securing the final point for Alpine.