Novak Djokovic had to fight off an upset bid from qualifier Ernests Gulbis, finally defeating the Latvian 5-7, 7-6 (9/7), 6-1 on Friday to reach the semi-finals of the Montreal Masters.
But Rafael Nadal, like Djokovic a former champion at the event, was unable to escape an on-form Kei Nishikori as the fourth seed sent out the Spaniard 6-2, 6-4, thanks in part to six double-faults from Nadal.
Nishikori is riding a wave of momentum from his hardcourt title last week in Washington and will next face the winner from second seed Andy Murray and French 10th-seeded holder Jo-Wilfried Tsonga.
Nishikori won his first match against Nadal after seven losses, taking just under 90 minutes to advance in his best Canadian showing.
"It took a long time to finally beat him, but I`m very happy to win my first match against Rafael," said Nishikori, ranked fourth on the ATP. "I`m quite pleased with my form from last week. I just hope I can keep it going and get another title."
Seventh seed Nadal was playing in Canada for the first time since winning his third title here in 2013; he now stands 28-7 in the country.
Djokovic found himself down a set and a break against the Gulbis whose form comes and goes without warning.
Gulbis, whose career best was a Roland Garros semi-final in 2014, has since gone into a funk in 2015, winning only eight matches since the start of the year.
But the outsider rose to the big occasion as he forced his world number one opponent to save two match points in the second-set tiebreaker of a thriller lasting two and a half hours.
Djokovic levelled at a set apiece and re-established order as he ran away with the third set for the win.
"Very lucky" was the message he scrawled on the camera lens during the traditional tennis match broadcast sign-off.
Djokovic is bidding for his fourth Canadian title and the win puts him into a semi-final against unseeded Jeremy Chardy.
Chardy booked his first career appearance in the semi-final of an ATP Masters 1000 event, struggling for three hours to defeat John Isner 6-7 (9/11), 7-6 (15/13), 7-6 (7/4).
Chardy overcame 32 aces from the big American to reach the final four.
He will need to quickly recover after a marathon which only concluded in early evening and threw the day`s schedule at least three hours behind.
The match, which began two hours late due to rain, lasted three hours and eight minutes.
Chardy fought through despite 32 aces from Isner, who found himself struggling in a 19-minute second-set tiebreaker.
The American 16th seed missed the chance to close out the contest in straight sets, blowing five match points. Chardy needed four set points to carry the tight battle into a deciding set.
But that one also went to a decider, with the French challenger earning two match points and converting on the first.
Chardy advanced with 12 aces and a single break of serve while saving break points on six of seven occasions.
"I`m very proud to reach my first Masters semi-final," said the 49th-ranked winner who levelled his season record at 18-18. "I was lucky to come back in the second.
"I`m really happy. I stayed really focus and really strong in my head the whole match."
The Frenchman has cast a spell over Isner in their series, winning all four of their meetings including the Roland Garros second round last May.
"It was a crazy match, we had the chance, both of us. We did a lot of good points. We were both serving really well. We were really close the whole match."