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 will try to do everything to obviously be ready."
Unseeded Serena Williams had no problems progressing, easing past Alona Bondarenko 6-0 6-3 in just 46 minutes.
Williams will now face Germany's Julia Georges in round two in Toronto, with Zheng awaiting the winner in the third round 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."
Clijsters, who has won the US Open for the last two years running, injured her right ankle in April and then withdrew from tournaments in Rome and Madrid with shoulder and wrist injuries.
Tuesday's match was her first match since returning from a foot injury suffered in the Netherlands in June which forced her to miss Wimbledon, and she now faces a race to be fit for the US Open, which starts on 29 August.
Williams, though, is feeling positive about her chances of being in peak condition in New York.
The 29-year-old is playing in just her fourth tournament back after missing nearly a year through injury and illness, but powered past Bondarenko in comfortable style - losing just four points in her first-set stroll.
She said: "I would hate to be Grand Slam-ready right now, seeing as how the US Open is not right now, so I'll try to peak at the right time.
"I want to get better on a few things. I definitely want to try and continue to do more and play harder. I am so happy to be back and every match is so fun for me. I just really enjoy it."
Ana Ivanovic also had a comfortable passage through, beating China's Zhang Shuai 6-1 6-1, while 11th seed Andrea Petkovic beat Canada's Eugenie Bouchard 6-2 6-2 and 13th seed Agnieszka Radwanska defeated Russian Elena Vesnina 6-4 6-1.
Caroline Wozniacki, Li Na and Maria Sharapova are all in action on Wednesday.