Iga Swiatek, of Poland, hits a return to Karolina Muchova (not in photograph), of the Czech Republic, during the National Bank Open tennis tournament in Montreal, on Aug 10, 2023. (PHOTO / AP)

Iga Swiatek overcame a lengthy rain delay and a stern test from Karolina Muchova to seal a 6-1 4-6 6-4 victory at the Canadian Open that will assure the Pole retains her number one ranking when the US Open warm-up event concludes.

Riding the momentum from her fourth title of the season last week at her home tournament, the Warsaw Open, Swiatek looked poised for easy passage breezing through five straight games breaking Muchova twice to claim the first set.

But in each of their two previous clashes the contest went three sets and the third encounter would be no different as the Czech charged ahead 4-1 in the second and then after a little wobble, coolly closed out the set to level the match.

Karolina Muchova of the Czech Republic hits a return against Iga Swiatek (not in photograph) of Poland on Day 4 during the National Bank Open at Stade IGA on Aug 10, 2023 in Montreal, Canada. (PHOTO / AFP)

Just as play was about to begin in the third, rain swept across Montreal sending players to the locker room.

Riding the momentum from her fourth title of the season last week at her home tournament, the Warsaw Open, Iga Swiatek looked poised for easy passage breezing through five straight games breaking Karolina Muchova twice to claim the first set

They returned and Swiatek broke to open the deciding set but rain once again forced a delay.

Later, Swiatek would hold for 2-0 and that would be all the cushion she would need to win the rematch of the French Open final, where Swiatek also prevailed for her fourth Grand Slam title.

"You have to find energy even though we've been here since 9 am," Swiatek said after the match, which took more than nine hours to complete.

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"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 win, Swiatek guaranteed that Aryna Sabalenka, whose round of 16 match was postponed until Friday due to the weather, will not overtake her at the top of the rankings come Monday.

Swiatek will face American qualifier Danielle Collins, who dispatched Canadian wild card Leylah Fernandez 6-2 6-3, for a spot in the semis.

Iga Swiatek of Poland serves against Karolina Muchova (not in photograph) of the Czech Republic on Day 4 during the National Bank Open at Stade IGA on Aug 10, 2023 in Montreal, Canada. (PHOTO / AFP)

Earlier, fourth seeded American Jessica Pegula romped into the last eight with a 6-4 6-0 thrashing of Italian Jasmine Paolini, who in five career meetings has failed to take a set off the American.

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The two women traded early breaks before Pegula seized control with a break to claim the opening set then shifted into top gear steamrolling through the second to ease into her 10th quarterfinal of the season second only to Swiatek.

Pegula will battle her doubles partner and compatriot Coco Gauff in the quarters after the teenager crushed Wimbledon champion Marketa Vondrousova 6-3 6-0 under the lights in Montreal.