Toronto: World number two Kim Clijsters withdrew from her opening match at the Toronto Cup with a partial tear of her left stomach muscle, casting doubt on whether she will be fit for this month`s US Open.
The tournament`s second seed said her left stomach muscle felt tight during her warmup earlier in the day and had the area taped in a bid to protect the muscle.
"During the match it just felt like it went worse," the four-times grand slam winner told reporters after the match. "I already had an ultrasound here on site and I have a partial tear in my left stomach muscle and a little bit of blood."
The 28-year-old Belgian, who retired while leading 6-3, 1-2 against Chinese qualifier Zheng Jie, shook her head after hitting a forehand. She then placed her hands on her stomach before waving to the crowd as she left the court.
Clijsters, in her first match since a second-round defeat at the Den Bosch Open in June where she suffered a foot injury, will get more treatment in the coming days and said it was too soon to rule her out for the year`s final grand slam.
"No, not yet," said Clijsters, who won the US Open in 2005, 2009 and 2010. "I still have a few weeks until then so will try to do everything to obviously be ready."
Zheng will now play the winner of a second-round matchup between Serena Williams and Germany`s Julia Goerges.
Williams, who won the Stanford Classic on July 31 in her third tournament back after missing nearly a year with injuries and health problems, barely broke a sweat as she powered past Alona Bondarenko 6-0, 6-3 in 46 minutes.
The American set the tone from the start when she book ended the first game with aces and surrendered a mere four points in the opening set.
"I definitely started out real good and I was excited about that," Williams said in an on-court interview.
"I am so happy to be back and every match is so fun for me and I just really enjoy it."
Clijsters`s retirement marked the second seeded player to bow out of the tournament on Tuesday and sixth in two days.
After intense rain delayed the start of Tuesday`s action by over three hours, 11th-seed Andrea Petkovic finally took to the center court and eased past Canada`s Eugenie Bouchard 6-2 6-2.
Poland`s Agnieska Radwanska, seeded 13th, powered her way past Russia`s Elena Vesnina 6-4 6-1 while 16th-seeded Slovak Dominika Cibulkova retired while leading 3-1 in the third set over Czech Iveta Benesova.
The USD 2-million tournament kicks into high gear on Wednesday when champion and world number one Caroline Wozniacki, French Open champion Li Na and Wimbledon runner-up Maria Sharapova will be among those in action.