Williams and Ivanovic out of Doha

Ivanovic required treatment during her match against Vera Zvonareva

Serena Williams and Ana Ivanovic have withdrawn from their matches at the Tour Championships in Doha.

A stomach injury forced Williams to pull out of Friday's round robin match against Elena Dementieva, who will now advance to the semi-finals.

Ivanovic was set to play match against Svetlana Kuznetsova but was forced to forfeit the match because of a virus.

The Serbian world number four had no chance of making the semi-finals as she had lost her opening two matches.

Her match against Kuznetsova would effectively have been a dead rubber as the Russian too had failed to register a win so far.

In Friday's remaining match, Jelena Jankovic plays Vera Zvonareva.

606: DEBATE Who do you think will win the Tour Championships?

Ivanovic, who turned 21 on Thursday, won her first Grand Slam title at the French Open in June and briefly topped the world rankings, but she has since lost 10 out of 20 matches and won only one title, two weeks ago in Linz.

"To tell the truth, I don't like to complain but ever since I came here I wasn't feeling really well," she said after losing to Zvonareva on Wednesday.

"I got a virus and I had pain in my throat, so from the first day I wasn't feeling so good. But, obviously, didn't want to talk about it and maybe give advantage to other players knowing that.

"But it was very hard for me to breathe and I was struggling. I was feeling very dizzy and light-headed, so it was very tough. I couldn't eat much for the last couple of days.

"You know, with the lack of energy, it was a little bit hard."