American Venus Williams, playing in her first tournament since losing in the first round of the French Open in late May, lost to 13th seed Kirsten Flipkens in three sets in opening round action at the $2.369 million Rogers Cup on Tuesday.
Williams, who has never won a match at this event in four appearances, took the first set in dominating fashion, but eventually fell to the Wimbledon semifinalist, 0-6, 6-4, 6-2.
Williams has been hampered by a lower back injury that has kept her on the sideline.
Third seed Agnieszka Radwanska of Poland, was a second-round winner on Tuesday, defeating Belgian Yanina Wickmayer, 6-2, 6-3, while German left- hander Angelique Kerber went by way of a second-round upset.
Surging Slovak Dominika Cibulkova came from behind to upset the eighth-seeded Kerber, 6-7 (0-7), 6-2, 7-5, on the hardcourts at Rexall Centre. Kerber dropped to a disappointing 4-5 over her last nine matches.
The diminutive Cibulkova was a hardcourt titlist in Stanford just two weeks ago.
In opening-round play on Day 2, 11th-seeded Russian Maria Kirilenko mauled Croatian qualifier Petra Martic 6-2, 6-1, 15th-seeded former world No. 1 Serb Jelena Jankovic held off Georgian qualifier Anna Tatishvili 7-6 (8-6), 4-6, 6-3, and 16th-seeded former top-ranked Serb Ana Ivanovic drubbed Taiwan's Su- Wei Hsieh 6-1, 6-2. The former U.S. Open finalist Jankovic was the Toronto runner-up in 2007.
Also on Tuesday, former U.S. Open champion Samantha Stosur beat Julia Glushko, 5-7, 6-2, 6-3, in opening-round play. The Aussie is fresh off her title in Carlsbad, where she upset former world No. 1 Victoria Azarenka in the final.
Also in the first round on Tuesday, American Lauren Davis surprised lucky loser and two-time Grand Slam champion Svetlana Kuznetsova of Russia, 3-6, 7-5, 7-5. Davis is rewarded with a second-round match against Wimbledon champ Marion Bartoli.
Additional opening-round wins came for France's Alize Cornet, Russian Ekaterina Makarova, Canada's Sharon Fichman, Spain's Carla Suarez Navarro, Canadian Eugenie Bouchard, Italian Francesca Schiavone and hot Slovak Magdalena Rybarikova, who beat Romanian Alexandra Dulgheru 6-4, 6-3. Rybarikova was last week's repeat titlist in Washington, D.C.
Czech Petra Kvitova beast Chinese Li Na in last year's Rogers Cup final in Montreal, as this event shifts between Toronto and Montreal each year.
The 2013 Rogers Cup titlist will claim $426,000.