MONTREAL (AFP) -- Iga Swiatek, unfazed by two lengthy rain delays, held off Karolina Muchova 6-1, 4-6, 6-4, to reach the quarterfinals of the WTA Montreal Open on Thursday, Aug. 10.

Swiatek's victory, in a rematch of the French Open final she won two months ago, ensures the Polish star will remain number one in the world for a 72nd straight week when the rankings come out on Monday.

It took two hours and 47 minutes on court, but more than nine hours from start to finish on what Swiatek called an "extraordinary" day.

Swiatek roared through the opening set despite solid play from her Czech opponent, who was unable to convert break chances in the third and fifth games.

Swiatek grabbed her second break of the set in a sixth game that went to deuce five times.

But as she did at Roland Garros, Muchova battled back to take the second set only for rain to bring the match to a halt for more than three hours.

Swiatek had broken to open the third set when rain stopped play again. It would be another couple of hours before they resumed and she backed up the break with a hold for a 2-0 lead.

That would be all Swiatek needed, and she polished off the victory with a love game.

"You have to find energy even though we've been here since 9 am," Swiatek said. "For sure, it was a pretty extraordinary day, and I don't think I've had such a situation in my career so there is the opportunity to learn something new and see what I'm capable (of) -- even though we played this match, like, three times."

Swiatek next faces either Canadian wild card Leylah Fernandez or US qualifier Danielle Collins.

Fourth-seeded American Jessica Pegula booked her quarterfinal berth with a crisp 6-4, 6-0 victory over Jasmine Paolini before the rain rolled in.

Pegula fired six aces and broke Paolini five times, needing just 69 minutes to post her fifth win in as many meetings with the Italian.

Pegula, who has reached the semifinals at the last two Canadian tournaments, next faces her doubles partner Coco Gauff, who blew past newly minted Wimbledon champion Marketa Vondrousova 6-3, 6-0 in just 62 minutes.

Gauff, who captured her fourth career title on Sunday in Washington, is seeded sixth.

Vondrousova was seeded ninth a month after she earned her first Grand Slam title.

Other winners Thursday included 10th-seeded Daria Kasatkina, who beat Czech Marie Bouzkova 6-3, 6-4.

Two other third-round matches were postponed until Friday, when second-seeded Aryna Sabalenka will take on Liudmila Samsonova and seventh-seeded Petra Kvitova plays Belinda Bencic.