World number two Kim Clijsters withdrew from her opening match at the Toronto Cup on Tuesday 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 warm-up 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 match-up 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 centre 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 $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.
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