Czech Linda Noskova lost on her first outing since winning the women's singles title at Wimbledon.
The sixth seed was beaten 7-6 (7-5) 6-1 by American Caty McNally in Toronto. Noskova had led 5-1 in the opening set and also had two set points in the eighth game before capitulating.
McNally will face Alexandra Eala, who won the Washington Open on Monday to become the first player from the Philippines to win a WTA singles title, in the last 32. Eala beat American Alycia Parks in three sets.
Men's top seed Alexander Zverev was also beaten on his first outing since Wimbledon, where he lost to Jannik Sinner in the men's singles final. The world number three lost to Tallon Griekspoor 6-7 (3-7) 6-2 6-4.
Elsewhere, men's fourth seed Daniil Medvedev suffered his own shock exit to a Dutch opponent, losing in straight sets to world number 70 Botic van de Zandschulp.
Elena Rybakina, Jessica Pegula, and Coco Gauff - seeded second, third and fourth - all advanced to the third round, although Rybakina was on court for more than two and a half hours as she outlasted Daria Kasatkina 6-3 5-7 6-4.
