Beijing: Reigning China Open champion Victoria Azarenka crashed out in the first round on Monday as the second seed faced stiff resistance from former finallist Andrea Petkovic.
The Belarusian world number two rarely showed glimpses of the form which helped her claim the title last year, at times appearing sluggish as she committed a staggering 15 double faults in the 6-4, 2-6, 6-4 loss.
It is the second early exit in a row for the 24-year-old, who complained of feeling ill when she was defeated by Venus Williams in the second round of the Pan Pacific Open last week.
Azarenka appeared agitated in the opening set in Beijing's National Tennis Centre, even before German Petkovic broke her serve following a string of double faults.
The second seed regained her composure in the second set and appeared to be on course for victory when she broke Petkovic at the start of the third set.
However, the 2011 finallist then seized the initiative, claiming victory in two hours and 22 minutes.
China's Zhang Shuai continued her sizzling form, defeating compatriot Peng Shuai 6-3, 6-3 to set up a second-round match against tenth seed Roberta Vinci.
Wildcard Zhang won her first WTA title in Guangzhou earlier this month.
The 24-year-old recruited her own coach in July, following Chinese star Li Na's example in leaving the tightly controlled state sports system, raising hopes that she can emulate the success of the 2011 French Open winner.
It was a successful day for three Serbian players.
Eighth seed Jelana Jankovic needed three sets to overcome Russian Anastasia Pavlyuchenkova 1-6, 6-4, 6-0. Compatriot Ana Ivanovic claimed victory over Italian Flavia Pennetta 7-6 (11/9), 6-1.
Ivanovic had been trailing 5-1 in the first set.
"I really struggled to find my rhythm in the beginning. I felt the ball was flying," she said.
"I didn't really think I could come back in the first set, but I really played well and stepped up when it was very tight, so that gave me confidence."
Bojana Jovanovski rounded off a hat-trick for the Serbians, beating Romanian Sorana Cirstea 6-3, 6-2.
Also in the first round, Canadian Eugenie Bouchard defeated Slovakia's Magdalena Rybarikova 6-4, 6-1 and Russia's Svetlana Kuznetsova swept aside Hsieh Su-Wei of Taiwan 6-1, 6-1.
The men's tournament got under way Monday, but it was heartbreak for the home crowd after China's top-ranked player fell at the first hurdle.
Zhang Ze was knocked out by Australian number one Bernard Tomic 7-6 (7/4), 6-4.
Tomic will now take on fifth seed Richard Gasquet.
"We were playing juniors matches before and when Tomic was a junior he was sometimes sloppy and he gave many free points," the 23-year-old Zhang said.
"Now he has grown a lot and he is more serious on every single point when he plays."