Kim Clijsters could miss out on the chance to defend her US Open title later this month after retiring against Zheng Jie at the Rogers Cup in Toronto. The Belgian was a set up when she withdrew with a stomach problem.
Asked about her prospects for Flushing Meadows, she said: "I still have a few weeks until then so I will try to do everything to be ready."
Unseeded Serena Williams had no problems progressing, easing past Alona Bondarenko 6-0, 6-3 in just 46 minutes. She will now face Germany's Julia Georges in the second round, with Zheng awaiting the winner after her unexpected progress.
Clijsters was leading 6-3, 1-2 when she pulled out, showing some discomfort after hitting a forehand, and she later revealed she felt pain in her stomach during the warm-up and had it taped ahead of the match.
She added: "During the match it just felt like it went worse. 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."