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Serena Williams and Ana Ivanovic have withdrawn from their matches at the Tour Championships in Doha.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.
Serbian world number four Ivanovic was scheduled to play against Svetlana Kuznetsova but was forced to forfeit the match because of a virus.
In Friday's other match, Vera Zvonareva beat Jelena Jankovic 2-6 6-3 6-4.
The semi-final line-up will see Zvonareva take on Dementieva, and Venus Williams face Jankovic.
Serena Williams had been in tears during her one-sided 5-7 6-1 6-0 defeat to sister Venus on Thursday and it emerged that she may have suffered her injury then.
"I had zero pain last night," Serena said. "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."
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.
She had no chance of making the semi-finals in Doha as she had lost her opening two matches.
And the match against Kuznetsova would effectively have been a dead rubber as the Russian too had failed to register a win.
"The other night was a very intensive and long match (against Zvonareva) and I started to feel actually worse," said Ivanovic.
"So, yesterday, I didn't feel good and today even worse. I tried to hit actually yesterday.
"I was on the court for about 10 minutes, and as long as I didn't move it was OK.
"But if I try to hit a little bit harder or to move on the side, it was impossible to do."