Injured Serena pulls out of Doha tournament

(CNN) -- Serena Williams' hopes of ending the year on a high note are over after she was forced to withdraw from the season-ending Sony Ericsson tournament in Doha with a stomach muscle problem.

Williams had been in tears during her one-sided 5-7 6-1 6-0 defeat to older sister Venus during their round robin match on Thursday -- and it has emerged she may have suffered the injury then.

"I had zero pain last night," Serena said of her post-match experience. "But I didn't serve well and I think that's why I was doing that. I might have been over-compensating.

"When I got home I didn't feel pain. But when I woke up I was in pain. I have had this before and so when I felt this it was like a red flag. I went to see how I felt in practice yesterday, and it was hard when I was hitting ground strokes. I didn't want to risk it."

The setback means that Williams is unable to take the chance of avenging herself for her Olympic defeat to Elena Dementieva, who went on to take the gold medal in Beijing in August.

Nadia Petrova has come in as the second alternate to take on her fellow- Russian instead, but Dementieva is already certain of qualifying for the semifinals, where she joins her compatriot Vera Zvonareva, Jelena Jankovic, and Venus.

It was the tournament's second major setback within two hours, after another former world number one, Ana Ivanovic, withdrew from her final match against Svetlana Kuznetsova with a virus.

Meanwhile, world number one Jankovic was beaten by in-from outsider Zvonareva, the last player to qualify for the original eight, on Friday.

The little-considered Russian's 2-6 6-3 6-4 surprise victory means that she remains unbeaten with three wins, finishes top of her group, and avoids facing Wimbledon champion Venus in Saturday's semifinals.

Instead, Zvonareva will meet her compatriot Dementieva, the Olympic champion, while Venus will play Jankovic -- with the winners to meet in Sunday's final.