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Reigning Wimbledon champion Marketa Vondrousova was pushed by former world number one Caroline Wozniacki but managed to hold on for a 6-2 7-5 second-round win in Montreal.
Wozniacki marked her return after more than three years away from the court with an opening-round victory over Kimberly Birrell, but looked outmatched early against the number nine seed.
Vondrousova dictated play in the first set, and looked set to coast to victory in the second as she served for a 4-0 lead.
However the Czech misfired, serving back-to-back double faults and opening the door for the 2018 Australian Open champion.
The 33-year-old took that opportunity to gain a 4-3 advantage and threaten a massive comeback.
But Vondrousova was ultimately able to right the ship and set up a meeting with Coco Gauff in the next round.
In the third round, there will be a rematch of the French Open final between world number one Iga Swiatek and Czech Karolina Muchova.
Playing her first match of the North American hard-court swing, Swiatek was not at her best in the opening set against Karolina Pliskova but pulled away to win 7-6 (6) 6-2.
Now she will try to engineer a repeat of her dramatic three-set win over Muchova in Paris, which earned her a fourth grand slam title.
Third seed Elena Rybakina survived a real test from a resurgent Jennifer Brady, coming through 6-7 (3) 7-6 (5) 6-3.
Former Australian Open finalist Brady has been dogged by injury for two years but has quickly hit her stride and was unlucky not to pull off a huge upset.
Sixth seed Gauff, who recently linked with Andy Murray’s former coach Brad Gilbert, continued her strong form with a 6-2 6-2 win against Katie Boulter while fourth seed Jessica Pegula saw off Yulia Putintseva 6-4 6-4.
Elsewhere, Aryna Sabalenka and Danielle Collins earned straight sets wins over Petra Martic and Maria Sakkari respectively, while Leylah Fernandez, Daria Kasatkina, Belinda Bencic and Liudmila Samsonova also progressed to the third round.