Anett Kontaveit spoiled Maria Sharapovas return, rallying to beat the five-time Grand Slam champion 4-6, 6-3, 6-4 in the opening round of the Rogers Cup on Monday.
Sharapova, competing for the first time since retiring from her first-round match at Wimbledon, appeared to be on course for her fourth match win since January when she went up a set and an early break against the 16th seed from Estonia.
Sharapova said Mondays physical battle showed shed made progress since Wimbledon.
German 12th seed Angelique Kerber was unable to make it to the second round, losing 0-6, 6-2, 6-4 to Russias Daria Kasatkina.
Important results (first round):
Women: Sofia Kenin bt Hsieh Su-Wei 6-4, 6-3; Belinda Bencic bt Anastasia Potapova 6-2, 6-1; Dayana Yastremska bt Johanna Konta 6-3, 6-2; Anett Kontaveit bt Maria Sharapova 4-6, 6-3, 6-4; Daria Kasatkina bt Angelique Kerber 0-6, 6-2, 6-4; Caroline Wozniacki bt Yulia Putintseva 6-4, 6-2.
Men: Gael Monfils bt Peter Polansky 6-7(4), 6-3, 6-3; Roberto Bautista bt Bernard Tomic 6-3, 6-2; Stan Wawrinka bt Grigor Dimitrov 6-4, 6-4.
Milos Raonic bt Lucas Pouille 6-4, 6-4; Jan-Lennard Struff bt Jo-Wilfried Tsonga 6-2, 6-2.