TORONTO (Reuters) -Romania's Simona Halep fought off a late rally from Coco Gauff to secure a nervy 6-4 7-6(2) win on Friday and advance to the Canadian Open semi-finals for a fifth time.
Twice champion Halep, up a set and leading 4-1 in the second, looked headed for an easy passage into the last four.
But Gauff dug in and rallied to force the second set to a tiebreak before 15th-seeded Halep finish off the 10th seed in the tiebreak to maintain her dominance over the American.
In four career meetings, including three this year, Gauff has yet to take a set off Halep, who next faces another American in seventh seed Jessica Pegula - a 6-3 6-3 winner over Kazakh Yulia Putintseva.
"I'm really pleased with the way I stayed there strong mentally," said Halep, after reaching her 29th career semi-final at a WTA 1000 event - more than any other player.
"And I fought for every point which was really important because she's doing the same thing. Mentally I'm strong even if I struggled a little bit in the second set."
Halep certainly struggled in the second as she twice served for the match at 5-4 and 6-5 only to be broken each time.
But the twice Grand Slam winner refocused in the tiebreak to stop the contest going to a third and registered her 36th win this season behind only Iga Swiatek (48) and Ons Jabeur (37).
The 30-year-old also had to wrestle with her serve, committing eight double faults while giving French Open finalist Gauff plenty of chances with 12 break opportunities.
(Reporting by Steve Keating in Toronto; Editing by Ken Ferris)