Victoria Azarenka's China Open title defence wilted as she was sent crashing out at the first hurdle by former finalist Andrea Petkovic 6-4 2-6 6-4.
The world No 2 rarely showed glimpses of the form which helped her claim the title last year, although she faced stiff resistance from the German in Beijing's National Tennis Centre.
Azarenka's 15 double faults, to go with 44 unforced errors, mirrored her erratic display and followed her second-round defeat by Venus Williams in Tokyo last week when she complained of feeling ill.
The 24-year-old Belarusian appeared agitated in the opening set, even before German Petkovic broke her serve following a string of double faults.
But Petkovic looked in discomfort in the second, needing a medical time out, and she went on to lose seven straight games as Azarenka forced the decider.
However, the 2011 finalist then seized the initiative in the third set, claiming victory in two hours and 22 minutes.
In the day's other matches, China's Zhang Shuai continued her fine form, defeating compatriot Peng Shuai 6-3 6-3 to set up a second-round match against tenth seed Roberta Vinci.
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 while compatriot Ana Ivanovic claimed victory over Italian Flavia Pennetta 7-6 (11/9) 6-1, despite 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.