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.
Just as play was about to begin in the third, rain swept across Montreal sending players to the locker room.
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 a.m.," Swiatek said after the match, which took more than nine hours to complete.
"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.
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.
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.
Alcaraz outlasts Hurkacz, Ruud stunned at Canadian Open
World number one Carlos Alcaraz produced a thrilling comeback to beat Hubert Hurkacz 3-6, 7-6 (2), 7-6 (3) on Thursday and reach the quarter-finals of the Canadian Open as the Wimbledon champion continues to refine his game ahead of the upcoming US Open.
The swashbuckling Spaniard got off to a nightmare start as Hurkacz jumped out to a 3-0 first set lead in just six minutes en route to capturing the first set.
Trouble mounted further for Alcaraz when he was broken at the start the second but he broke back for 1-1 and from there began to pick up the pace on his forehand while the Pole's serving cooled off.
Alcaraz dominated the second set tiebreak and appeared in complete control when he whipped a sensational crosscourt forehand winner for a 5-2 lead in the decider that brought the Toronto crowd to their feet.
But the natural showman got a little too cute in the next game, attempting back-to-back drop shots on match points that landed in the net and he was ultimately broken.
Hurkacz began to find his form again, breaking at love for 5-5 and holding to take a 6-5 advantage.
But there would be no escape from Alcaraz in the third set breaker where Hurkacz tightened up and committed some costly errors including a forehand that sailed long to deliver Alcaraz his 14th straight win.
Up next for Alcaraz is a quarter-final with one of two Americans - Tommy Paul or Marcos Giron.
Soon after the conclusion of the match, Scot Andy Murray announced that he had to pull out of his round of 16 match against Italy's Jannik Sinner with an abdominal injury.
Earlier, Alejandro Davidovich Fokina battled back from a third set deficit to beat Casper Ruud 7-6(4) 4-6 7-6(4) and deny the third seed a place in the Canadian Open quarter-finals.
Ruud was two points away from victory at 5-3 30-0 before Davidovich Fokina served his way out of trouble.
Still leading 5-4, the Norwegian had a chance to seal the match but was unable to do so, badly shanking a shot on break point to give new life to his opponent.
Both players held serve to send the contest to a tiebreak as the match crossed the three-hour mark.
Ruud dug himself into an early hole in the breaker with some tentative serving and poor forehands and Davidovich Fokina claimed the win with a drop shot.
"When I broke at 5-4, that gave me like a shot of tequila," the Spaniard said.
Up next for Davidovich Fokina is American Mackenzie McDonald, who reached his first quarter-final of a Masters 1000 tournament by dispatching Canadian Milos Raonic 6-3 6-3.
After crushing 52 aces across his first two matches, hometown hero Raonic's biggest weapon proved his undoing.
He managed just nine aces to go with six double faults, some coming at the worst possible times including on set point in the opener and a break point in the second that put McDonald up 2-0.
Despite the loss, Raonic, a former world number three who returned to the tour in June after a two-year absence due to injury, said the tournament was a positive experience.
"A lot of things to look back and be proud of. Happy with how the week went overall" he said.
"I wish I could have been better on court today but you can't discredit what two years away does, right?"
Australia's Alex De Minaur beat eighth seed Taylor Fritz 7-6 (7), 4-6, 6-1 in a topsy-turvy battle to reach the quarters of a Masters 1000 tournament for his first time.
Fritz led 5-1 in the opener before De Minaur battled back with help from 27 unforced errors in the set from the American, and De Minaur converted his sixth set point in the tiebreak.
Fritz regrouped and rode an early break in the second set to level the match but De Minaur was razor sharp in the decider, using his all-court speed and gritty style to jump out to a 2-0 lead he would not relinquish.
De Minaur will next face a familiar foe in the quarter-final in the form of second seed Daniil Medvedev, who cruised past Italy's Lorenzo Musetti 6-4, 6-4.
Medvedev holds a 4-1 lead in head-to-head matchups with De Minaur.
