Spain's reigning MotoGP champion claimed his second sprint victory of the season, beating teammate Francesco Bagnaia for a Ducati one-two while VR46 Racing's Franco Morbidelli completed the podium after starting 18th on the grid.
The sprint began under cloudy skies before rain transformed the race into a lottery as several riders crashed while others scrambled to the pit lane to switch to bikes with wet-weather tyres as conditions deteriorated.
'First victory with a crash'
"It's my first victory with a crash and it's true that I crashed in the best corner, in the best moment," a relieved Marquez said after he moved up to fourth in the standings, 24 points behind championship leader Marco Bezzecchi.
"At that lap, I was thinking of going in (to pit) but then Alex (Marquez) was leading the race and he stayed out. I made the mistake of following him."
After securing his first pole of the season earlier on Saturday, Marquez shot off the line and took the lead into turn one ahead of Johann Zarco, with his brother Alex moving up to third.
Marc built more than half a second's advantage as Aprilia's Jorge Martin attacked Alex to claim third place.
But Martin was struggling with a front brake disc that was glowing red-hot after two laps, forcing him wide before eventually retiring as he nursed his bike back to the pits.
Alex finally overtook Zarco after two laps and set his sights on his elder brother. Alex eventually reeled in Marc and made his move to take the lead as spots of rain began falling.
Marc Marquez crashes near pit lane
The drama intensified with four laps to go when Marc lost control and crashed on the final corner.
Ducati team manager Davide Tardozzi had a clear view of Marquez crashing at the final turn and immediately signalled his team to prepare the spare bike as the Spaniard immediately pit in to swap for a bike with wet tyres.
"I don't know, somebody today gave me that extra luck that I crashed on that last corner," Marquez added.
"I just waited until everybody passed and then I knew that was the only chance to put the rain tyres on that lap immediately."
Alex opted to stay out but as conditions became treacherous he too crashed out of the lead when the heavens opened with three laps left, prompting more riders to pit and swap bikes in chaotic scenes.
Bezzecchi was among those who crashed, leaving the two Ducatis of new race leader Bagnaia and Marc to fight for victory.
Marc eventually seized his moment, diving inside his teammate to take the lead and claim an extraordinary win but Bagnaia was all smiles as he secured his second podium of the season after starting 10th on the grid.
"It's a great result, thanks to the team for their job and today the gamble was on point. So happy," he said.