TORONTO // Kim Clijsters' US Open title defence is in doubt after she pulled out of the WTA Toronto hard court tournament last night 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.
Clijsters said she still hoped to defend her title at the US Open, the final grand slam of the season that starts on August 29 in New York.
"I still have a few weeks till [the US Open] and we'll try to do everything to be ready," said Clijsters, who delivered an unusual six double faults before calling it a day after 54 minutes on court.
"During the match it just felt like it went worse," Clijsters said.
"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."
Serena Williams, unseeded here, continued her own injury comeback in emphatic style, crushing the Ukrainian Alona Bondarenko 6-0, 6-3 to book a second round berth.
The American lost just four points as she won the first set in 16 minutes.
And after Bondarenko kept it on serve to reach 3-3 in the second, Williams again hit high gear, winning three games to secure the victory.
Williams is rebuilding a ranking that plummeted after she was sidelined by injury and illness for 11 months.
The American has already shown she will be a force to be reckoned with, capturing the Stanford title two weeks ago.
But despite the ease of her victory, she stopped short of saying she is grand slam-ready.
"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," Williams said.
"I want to get better on a few things. I definitely want to try and continue to do more and play harder."
Williams plays Germany's Julia Goerges in the second round for a place against Zheng in the third.
Top-seeded Carolina Wozniacki of Denmark is set to open her campaign tonight with a second-round match against Italy's Roberta Vinci, who won a first round 6-4, 6-2 victory over Belgian Yanina Wickmayer.
Serbia's Bojana Jovanovski booked a meeting with fifth-seeded Russian Maria Sharapova after Australian Jelena Dokic retired with a right shoulder injury while trailing 0-2.
In other first-round matches, 11th-seeded Andrea Petkovic advanced with a 6-2, 6-2 victory over Canadian Eugenie Bouchard.
Poland's 13th-seeded Agnieszka Radwanska defeated Russian Elena Vesnina 6-4, 6-1, but 16th-seeded Slovakian Dominika Cibulkova had to retire against Czech qualifier Iveta Benesova while leading 3-1 in the third set due to a left abdominal strain.