MONTREAL — Top-ranked Iga Swiatek of Poland beat 14th-seeded Karolina Muchova of the Czech Republic 6-1, 4-6, 6-4 on Thursday night in the National Bank Open in a match interrupted by rain for more than six hours.

Coming off her fourth victory of the year last week at home in Warsaw, Swiatek finished off Muchova after they were delayed for three hours, 20 minutes following the second set and for two hours, 50 minutes early in the third.

“You have to find energy, even though we’ve been here since 9 a.m.,” Swiatek said in her on-court interview. “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 — even though we played this match like three times.”

With the victory, Swiatek — who also beat Muchova in June in the French Open final — guaranteed she’ll will remain No. 1 for 72nd straight week when the new ranking comes out Monday. The 22-year-old player leads the tour with 49 match victories.

Swiatek set up a quarterfinal match against American Danielle Collins, who beat hometown favorite Leylah Fernandez 6-3, 6-3 in the final night match.

In the only match completed in the afternoon, fourth-seeded Jessica Pegula of the United States beat Italy’s Jasmine Paolini 6-4, 6-0.

Pegula will face sixth-seeded double partner Coco Gauff, the fellow American who beat ninth-seeded Wimbledon champion Marketa Vondrousova of the Czech Republic 6-3, 6-0. Gauff won last week in Washington for her fourth career title.

Third-seeded Elena Rybakina of Kazakhstan beat American Sloane Stephens 6-3, 6-3. Rybakina will face No. 10 Daria Kasatkina of Russia, a 6-3, 6-4 winner over Marie Bouzkova of the Czech Republic.

The match between No. 2 Aryna Sabalenka of Belarus and No. 15 Liudmila Samsonova of Russia was pushed back to Friday. Seventh-seeded Petra Kvitova of the Czech Republic and 12th-seeded Belinda Bencic of Switzerland also will play their third-round match Friday.