Serena Williams and Maria Sharapova secured semi-final spots in the season-ending WTA Tour Championships in Istanbul with hard-fought victories.
Williams, the Wilmbledon and U.S. Open champion, was made to toil by Chinese Li Na, before prevailing over her 7-6 (2), 6-3.
"I started out really strong in the first game but then started making errors, and Li started playing her game and playing it well," Williams said after the match. "I was a little tight and my serve wasn't working at all today.
"The good thing is I can only play better."
Williams had already won her first match in the Red Group against Angelique Kerber, meaning the third match against Victoria Azarenka will not make much of a difference for the American.
Azarenka started her campaign with a marathon 6-7 (11), 7-6 (2), 6-4 win against Kerber. One more win for the Belarusian will ensure she ends the year as the world number one.
"I was fighting so hard -- at the beginning of the tournament it's quite hard to have a match like that," Azarenka said. "But all the best players are here and you have to show your best tennis.
"I don't think I did that, but at least I showed my best fight. It was difficult, but I'm glad I lived this experience and overcame all the challenges I had today.
"On the two match points against me I felt I had to go for it," Azarenka added. "I felt she would not miss, so I didn't want to wait for her to make a mistake -- I was determined to make it happen.
"I wanted to change the momentum and take destiny into my own hands. I had a clear mind of what I had to do. It takes time to go through to learn that."
The only person that can take the world number ranking from Azarenka - Maria Sharapova - also sealed a place in the semis with another tough three-set 5-7, 7-5, 7-5 win over Agnieszka Radwanska in the White Group.
"It was a very physical match," Sharapova said. "It just came down just to a few points in the end. I think today was an example of fighting through it, hanging in there, and getting it done.
"Her capability of anticipating where the ball goes and getting it back is amazing. No matter if it's a hard shot or a lower slice that has no pace, she makes you work for it.
"She did that right until the end. I found a way to get through those points against her, which is not always easy, especially on a slower court like this, but I managed."
Meanwhile, defending champion Petra Kvitova withdrew from the tournament citing illness, with Australian Sam Stosur taking her place in the tournament.
"I have to say sorry for the crisis," Kvitova said. "I have a stomach virus.
"Yesterday when I played I thought that it was nerves, but normally nerves are gone after the first set and it was actually during the whole match, so I thought that something was wrong."
