British No 1 Johanna Konta has been forced to withdraw from her last-16 match against Coco Gauff at the National Bank Open in Montreal due to a left knee injury.
Konta was making her return to action in Canada after withdrawing from Wimbledon as a close contact of someone with a positive case of Covid-19, and subsequently contracted the virus herself to miss the Olympic Games in Tokyo.
After progressing through the first round against Zhang Shuai when her opponent retired with a leg injury, the Briton rallied from a set down to upset third-seed Elina Svitolina.
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But she handed 17-year-old Gauff a passage through to the quarter-finals via a walkover on Thursday after withdrawing in advance of her clash with the 15th seed.
Konta is now a doubt for next week's Western & Southern Open in Cincinnati and the US Open, which begins on August 30.
Top-seeded Aryna Sabalenka ended Rebecca Marinos strong run with a 6-1 6-3 victory over the Canadian.
Marino, a wildcard ranked 220th, made it to the round of 16 by upsetting No 16 seed Madison Keys and Paula Badosa, ranked No 31.
But the No 3 seed came through in under an hour to reach her eighth quarter-final of the year.
Marino, the former world No 38 is returning to the sport after retiring in 2013 due to depression.
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In the men's event taking place in Toronto, Stefanos Tsitsipas celebrated his 23rd birthday by earning a spot in the quarter-finals with a 6-3 6-2 victory over Karen Khachanov.
The 2019 ATP Finals champion will next play red-hot Norwegian Casper Ruud, who defeated Serbia's Dusan Lajovic 6-4 6-3.
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