Three players will be fighting to be World No1 after Caroline Wozniacki stumbled out of the Australian Open yesterday.
The Dane fell to defending champion Kim Clijsters and, as a result, will have to surrender top spot.
Russian Maria Sharapova, Czech Petra Kvitova and Victoria Azarenka of Belarus are in contention to inherit the prized position.
Clijsters' 6-3, 7-6 (7-4) victory over Wozniacki propelled her into a semifinal showdown with Azarenka.
The third seed had earlier scraped past Pole Agnieszka Radwanska in three sets, 6-7 (0-7), 6-0, 6-2.
Clijsters was headed for an easy win when she was up a set and a break before she found herself suddenly facing stiff resistance.
"It definitely didn't feel like I was up a set and 5-2," Clijsters said.
"Caroline is a great player and fighter and I think she changed her tactics and upped her aggression.
"I was happy to get through not in three sets because it is so hot."
Azarenka put down her comeback win to a change in mentality. "I think I am a different player right now, especially mentally-wise," she said.
"Maybe two years ago I would be like, okay, it's not working today. I'm going to try, but we'll see how it goes.
"Today, I really tried to forget about the first set and start from zero and really fight hard, really take it one at a time and keep going."
Azarenka was quick to point out she was still trying to become consistently strong mentally.
"I think that's the toughest thing out there. Especially, weather is tough. Your mind is sometimes boiling and it's difficult to think.
"Plus you have somebody on the opposite side that is making you a little bit crazy, and you have to keep it together. But it's a learning process. It's taking a lot of time.
"I think it's the hardest thing to do out there."
AAPBy Robert Grant