DOHA, Qatar — Serena Williams and Ana Ivanovic withdrew from the WTA's season-ending event on Friday, when top-ranked Jelena Jankovic advanced despite losing 2-6, 6-3, 6-4 to Vera Zvonareva.

Zvonareva and Jankovic both advanced to the semifinals of the Sony Ericsson Championships because of winning records in the round-robin stage of the tournament.

Zvonareva plays Elena Dementieva and Jankovic faces Venus Williams on Saturday.

Serena Williams pulled out with a stomach injury before her match against Dementieva, handing the Russian a spot in the final four.

"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 Thursday.

Earlier, Ivanovic withdrew from her match against Svetlana Kuznetsova because of a virus. The players' losing records meant neither had a chance of advancing to the semifinals.

Jankovic said she did not know the circumstances of the withdrawals and could not comment. She noted that she also has ailments, including a back problem that requires painkillers before she plays.

"This is the last tournament of the year," she said. "I also have problems, but I'm willing to try my best."

Williams bristled and pointed a finger at a journalist who suggested she should have played Friday despite her injury.

"I should make you work out ... for like 22 years and make you have a severe stomach strain, and then tell you to play for the sponsors," she said. "You should try it."

The world's top eight players were divided into two groups, with the top two in each group advancing to the elimination stage.

The ninth-ranked Zvonareva is the lowest-ranked player in the eight-woman field but put on a poised performance.

She broke Jankovic at 3-3 in the third set, pressuring the Serbian with deep groundstrokes and occasional trips to the net. The Russian improved to 4-6 against Jankovic.

Williams declined to say whether she would play next year at Indian Wells, California, one of the Tour's nine top tournaments. The Williams sisters haven't played there since 2001, when the family was booed after Venus withdrew just before a semifinal match against Serena.

"We'll see. Who knows," Serena said. "We've had a lot of discussions with a lot of people. I don't want to get into that right now. I just want to keep focused on other things."

Under a restructuring of the WTA calendar and tournaments, top-10 players who miss the top tournaments could be suspended, though players can avoid the punishment if they make promotional appearances.