Madrid, Spain:Defending champion Justine Henin secured a place in the semifinals and Ana Ivanovic moved closer to the last four after both won at the Sony Ericsson Championships on Wednesday.
Former champion Serena Williams retired with a left knee injury in her opener after losing the first set to Anna Chakvetadze of Russia 6-4, and immediately went to a hospital.
Williams, the Australian Open champion, will undergo an MRI before deciding if she can continue playing at the WTA's season-ending championships.
Top-ranked Henin beat third-ranked Jelena Jankovic 6-2, 6-2 and, with Williams' retirement, advanced to the last four for a third straight time.
Ivanovic earned her second win and eliminated Daniela Hantuchova 6-2, 7-6 (9).
Henin broke Jankovic six times to win her 22nd successive match, and improve to 9-0 against the Serb.
"From the first point to the end, I took all of my chances," Henin said. "It was her first match, so maybe I had a little advantage because I played last night."
Henin broke in the first game, Jankovic leveled at 2-2, but the Belgian came to the net to break twice more to close out the first set against the first-time qualifier.
Although Jankovic had moments in the second set, Henin broke in the first and third games, and improved to 11-1 indoors this year.
"I couldn't really find my game at all, I couldn't really do anything," Jankovic said. "So many things wrong. I have to somehow stay positive because I'm still in the game. I still have two matches to go."
Williams suffered swelling in her left knee, the same one she had surgery on four years ago, and which also forced her to withdraw from 10 events in 2006.
Williams said she felt tightness in both knees before the match, and as the match progressed the left joint "got worse."
"It's beyond unfortunate this has happened and I can't even stress how disappointed I am," Williams said in a statement. "It's my intention to play on in this event, I have been training really hard, I came over to Europe ... with the goal in mind of doing really well here."
Before the ninth game, the tournament doctor treated Williams, who has reached the final in all three previous appearances at the WTA Championships. The 2001 champ played on but seventh-ranked Chakvetadze had little problem serving out.
If Williams is not fit to face Henin on Thursday, Wimbledon finalist Marion Bartoli of France will take her place for the last two matches of the round robin.
Ivanovic broke Hantuchova three times in rushing to 5-0 in the first set.
But just as she did in her victory over Svetlana Kuznetsova on Tuesday, Ivanovic let her opponent rally in the second set, where ninth-ranked Hantuchova led 5-4.
Ivanovic held her serve before both players exchanged breaks again to send it to a tiebreaker.
Hantuchova dropped three set points with netted shots, then sent another forehand into the net - her 38th unforced error - on Ivanovic's third match point.
"It was really nerve wracking but I'm very happy to win another match tonight," Ivanovic said. "After two late nights I can use the day off tomorrow."
On Thursday, 2004 champ Maria Sharapova can clinch her and Ivanovic's place in the semis with a victory over Kuznetsova, while Jankovic plays Chakvetadze.