"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 from Limburg province, 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 U.S. Open in 2005, 2009 and 2010. "I still have a few weeks until then so will try to do everything to obviously be ready."