TORONTO - Kim Clijsters' US Open title defence is in doubt after she pulled out of the WTA Toronto hardcourt tournament on Tuesday with a stomach muscle injury.

Clijsters was leading 6-3, 1-2 in her second-round match against China's Zheng Jie when she hit a forehand, shook her head and departed.

It was another setback for the Australian Open champion from Belgium, who injured her right ankle in April and then pulled out of tournaments in Rome and Madrid with right shoulder and wrist injuries.