Venus Williams got off to a flying start in her return from a two-month layoff but her game quickly fell apart as the seven-time Grand Slam champion slumped to a 0-6, 6-4, 6-2 loss to Kirsten Flipkens in the Rogers Cup first round on Tuesday.
Seeds Maria Kirilenko (11) Jelena Jankovic (15) and Ana Ivanovic (16) also had first round wins.
In the second round, Slovak Dominika Cibulkova upended eighth seed Angelique Kerber of Germany 6-7(0), 6-2, 7-5 and second seed Agnieszka Radwanska of Poland won 6-2, 6-3 over Belgium's Yanina Wickmayer.
Williams, who has played sporadically this season due to a back injury and had not competed since losing in the first round of the French Open in late May, started in fine form on Tuesday but gradually fell apart as the contest wore on.
"I just really started serving a lot more in the last week, so I'm not really 100 percent on the serve yet," the 33-year-old Williams said.
"So it was better not to take too many risks and just do something I felt more comfortable with. This week I will definitely be practicing my serve a lot more and getting more confident in it."
Williams has also been battling an auto-immune disease for the past few years and said she might have come back a week too early. But she feels she needs matches before the US Open, which begins in a little less than three weeks time.
World No. 1 Novak Djokovic powered to a comprehensive 6-2, 6-1 win over Florian Mayer in his first match since Wimbledon while Canadian players upstaged Rafael Nadal with a string of surprise victories.
The return of the two-time defending champion after a month off had followed Nadal's first competitive match since his shock first round exit at Wimbledon.
It proved to be a successful, yet tense, one for Nadal as he and his doubles partner Pablo Andujar saved two match points before they beat Spanish compatriots David Ferrer and Feliciano Lopez 6-7(2), 6-1, 12-10.
The second day's play, however, belonged to the understated locals, including three wildcard entries who stunned their more highly fancied opponents as they basked in the home support.
World number 71 Vasek Pospisil produced the biggest upset when he surged to an unlikely first round win over in-form American John Isner.
Pospisil came from a set and a break down to claim a 5-7, 7-6(5), 7-6(4) victory in bright and sunny conditions, thrilling a parochial home crowd who cheered his every winner.
Frank Dancevic, another local wildcard, also rallied to beat Taiwan's Lu Yen-Hsun in three sets while Felip Peliwo became the fourth Canadian to reach the last 32 when Finland's Jarko Nieminen retired with a pulled hamstring trailing 3-1 in the deciding set.
Canada's top ranked player Milos Raonic then completed a memorable day for the hosts with a 6-3, 4-6, 7-5 triumph over Jeremy Chardy of France to become the fifth local into the second round.
Image: Venus Williams
Photograph: Mark Blinch/Reuters