Kim Clijsters suffered a worrying injury setback as the second seed abandoned her opening match at the Rogers Cup in Toronto last night.
Less than three weeks before she is due to begin the defence of her US Open title in New York, and after two months out with a foot injury which caused her to miss Wimbledon, the 28-year-old Belgian pulled out of her clash with China's Jie Zheng soon after winning the opening set.
Clijsters, seeded second, took the set 6-3 and was 2-1 behind in the next when she withdrew due to a left abdominal injury.
Having been handed a first-round bye, it was Clijsters' opening match in the tournament, and she now faces a race to be fit in time for Flushing Meadows.
Former US Open champion Serena Williams looks increasingly prepared to mount a serious challenge for another grand slam, and began her Rogers Cup campaign with a 6-0 6-3 success over Ukraine's Alona Bondarenko in the first round.
The American triumphed at the recent Bank of the West Classic, her last tournament, and is looking to stretch her winning run.
She is ranked 80th in the world, after playing little tennis in the past year due to injuries, but could be the player to beat this week.
Among the seeds, Germany's 11th seed Andrea Petkovic crushed Canadian 17-year-old Eugenie Bouchard 6-2 6-2 in the first round, and Polish 13th seed Agnieszka Radwanska beat Elena Vesnina of Russia 6-4 6-1, but Slovakian 16th seed Dominika Cibulkova went out.
Cibulkova was taking on Czech player Iveta Benesova and dropped the first set 7-5 before taking the next 6-4. She led 3-1 in the decider but suffered an injury similar to the one Clijsters would later be affected by, and withdrew.
Australian Jelena Dokic was another player to retire injured, when trailing Serbian Bojana Jovanovski 2-0 in their opening set.
Other winners included Slovakian Daniela Hantuchova, who beat Gisela Dulko of Argentina in straight sets, and Serbia's Ana Ivanovic, who hammered China's Shuai Zhang 6-1 6-1.
Italians Flavia Pennetta and Roberta Vinci also went through to the second round, along with Canadians Stephanie Dubois and Aleksandra Wozniak, China's Shuai Peng, Spain's Anabel Medina Garrigues and Croatian Petra Martic.