Ivanovic is the reigning French Open champion
Top seed Ana Ivanovic has pulled out of the Olympic tennis tournament because of a thumb injury.
"This is one of the worst moments of my career," said the Serb, who now faces a battle to make the US Open, which starts on 25 August.
Ivanovic, who will be replaced by compatriot Jelena Jankovic as world number one on Monday, had not been able to practise since arriving in China.
She sustained the injury in training after losing at Wimbledon.
"For the past month I've been receiving so much treatment and I've been doing everything to recover after Montreal where I really wasn't able to play," she said.
"After there it was so painful. Yesterday we flew an osteopath in from Sydney to help with my hand.
It was one of the hardest decisions of my career
"I've had really great doctors to help and we've tried all kind of treatments.
"Nothing could get my hand ready to play and it was one of the hardest decisions of my career."
The French Open champion joins former Olympic champion Lindsay Davenport, Maria Sharapova, Tatiana Golovin, Amelie Mauresmo and Mary Pierce in missing the event.
The 32-year-old Davenport, who won gold in 1996, has struggled with a knee problem since Wimbledon but is expected to play in the doubles.
Second seed Jankovic, due to play Zimbabwe's Cara Black in the first round, is still a doubtful starter after picking up a calf strain.
Ivanovic's pull-out came on a rain-hit day at the tennis tournament where only nine of a scheduled 45 matches were completed.