Serena Williams and Ana Ivanovic have withdrawn from the season-ending Sony Ericsson WTA Tour Championship in Doha.

American Williams was set to take on Elena Dementieva on Friday, with the winner set to progress to the semi-finals, but pulled out with a stomach muscle injury.

Williams, who lost 7-5 1-6 0-6 to sister Venus on Thursday, said: "Once I felt the pain I was like, "red flags". I'm having a great time, a great career. I'm doing so well, it's not worth it."

She added: "When I practised I was in pain when I played the groundstrokes so I didn't want to risk it."

Serbian Ivanovic also withdrew before her final round-robin game against Svetlana Kuznetsova.

A statement on the French Open champion's website read: "Ana has withdrawn from the Sony Ericsson Championships due to a virus."

Ivanovic was on her way out of the tournament anyway as he had lost her first two matches to Jelena Jankovic and Vera Zvonareva.

Zvonareva capped a surprising run at the Khalifa International Tennis Complex on Friday by securing an impressive 2-6 6-3 6-4 victory over Jankovic to claim top spot in the White Group.

The lowest seed in the eight-player field, Zvonareva finished the group stage with a perfect 3-0 record, a complete reversal of her first appearance at this event when she went winless.

Zvonareva will face fellow Russian Dementieva in the semi-finals, while Venus Williams will meet world number one Jankovic.