DOHA, Qatar – Serena Williams and Ana Ivanovic withdrew from the WTA Tour's season-ending event on Friday before their last round-robin matches.Williams cited a stomach injury when she pulled out of her match at the Sony Ericsson Championships against Elena Dementieva, who advanced to the semifinals.
"I did feel pain last night," the U.S. Open champion said. "I was still in pain just getting out of bed."
Williams lost to her sister, Venus, 5-7, 6-1, 6-0 on Thursday.
Serena said she might have tweaked a muscle and that doctors had told her to rest.
Ivanovic withdrew earlier Friday from her match against Svetlana Kuznetsova, citing a virus.
The French Open champion had no chance of advancing to the semifinals because she had lost two matches in the group stage.
Kuznetsova had also lost two matches and had no chance of advancing.
"It's obviously very hard for me," said Ivanovic, who turned 21 on Thursday. "Yesterday I didn't feel good and today, even worse."
The tournament features eight players divided into two groups, with the top two in each group advancing to the elimination stage.