Venus Williams lost in her first match in more than two months, falling 0-6, 6-4, 6-2 to Kirsten Flipkens at the Rogers Cup on Tuesday.
It was Williams' first tour match since she lost in the first round of the French Open on May 26. The 33-year-old missed Wimbledon, after 16 consecutive appearances, because of a lower back injury.
"I wasn't really expecting to play that well in the first set," Williams said. "In the second, I just didn't handle it well. But I was pretty close there to closing that match out and then got down in the third."
Maria Kirilenko and Ana Ivanovic won in straight sets to advance to the second round. Kirilenko cruised past qualifier Petra Martic 6-2, 6-1, and Ivanovic defeated Su-Wei Hsieh 6-1, 6-2.
In other matches, American qualifier Lauren Davis edged Svetlana Kuznetsova 3-6, 7-5, 7-5; Dominika Cibulkova stopped Angelique Kerber 6-7 (0), 6-2, 7-5; and Canada's Eugenie Bouchard beat Alisa Kleybanova 6-3, 6-1.
Cibulkova needed nearly 3 hours to win the match.
"My serve was not working like I was expecting," she said. "But in the end I made it, so I was really happy."
Bouchard, a 19-year-old from Montreal, mixed up her effective ground strokes with some slick cut shots that left her Russian opponent stymied. There were more service breaks than holds early in the 79-minute match, which forced the Canadian to adjust her style.
"I just decided to go for it and started to play aggressive," Bouchard said. "That definitely helped me."
Jelena Jankovic needed three sets to defeat qualifier Anna Tatishvili 7-6 (6), 4-6, 6-3.
Ekaterina Makarova beat Anastasia Rodionova 6-1, 7-5, and Alize Cornet topped Elena Vesnina 6-3, 6-3.
American Sloane Stephens made her debut at the Rogers Cup with a 6-2, 7-6 (5) victory over France's Kristina Mladenovic in the opening round.
The 17th-ranked Stephens will face Germany's Mona Barthel on Wednesday. Serena Williams is the top seed at the $2.3 million tournament.
Also, in a match between Canadians, Toronto's Sharon Fichman advanced with a 5-7, 6-2, 6-2 win over Stephanie Dubois.
In other matches, Magdalena Rybarikova of Slovakia defeated Alexandra Dulgheru of Romania 6-4, 6-3; Italy's Francesca Schiavone defeated South Africa's Chanelle Scheepers 2-6, 6-3, 6-3; third-seeded Agnieszka Radwanska of Poland beat Belgium's Yanina Wickmayer 6-2, 6-3 while Sam Stosur, the No. 12 seed from Australia, defeated Julia Glushko of Israel 5-7, 6-2, 6-3.