(CNN) -- Victoria Azarenka toppled Li Na in their WTA Championships playoff to book a semifinal clash with Maria Sharapova and ensure she will end 2012 as the world No. 1.
Azarenka needed to win to stay in the season-ending tournament after tasting defeat at the hands of Serena Williams on Thursday and triumphed 7-6 6-3 over Li, the No. 8 seed from China.
The 23-year-old will now face Russia's Sharapova, who won her third straight match in Istanbul earlier on Friday, against already eliminated Sam Stosur, from Australia.
"It feels great, a dream come true," Azarenka said of her achievement in a court side interview.
"I want to play my last matches here and then enjoy being the world number one. I want to focus on this competition, there are still lots of challenges ahead.
"All the players are on top of their game and it's great to be part of the women's game doing so well at the moment."
Sharapova triumphed 6-0 6-3 against Stosur, surrendering just five points in an opening set that lasted only 21 minutes.
"After a really long match in my previous round, I was just trying to be really aggressive today and not spend as much time as I did the other day," Sharapova told the WTA Tour's official website.
"I knew coming in I was already going to be in the semis, but I didn't want to lose. I wanted to do the right things today and I did them."
Sharapova rejected the notion the match was a dead rubber, with her passage to the final four already secured, as well as Stosur's exit.
"They're not so dead -- you're playing in front of thousands of people," she said. "There are things you play for. I don't want to just play tennis -- I play to win.
"I wanted to try to replicate a match that matters -- in my mind, it does. I never want to go into a match when I'm in a competitive tournament atmosphere and feel like there's nothing on the line. That would never get me going."
In the evening's other match Agnieszka Radwanska, the No. 4 seed from Poland, booked a semifinal with Serena Williams after recording an epic 6-7 7-5 6-4 victory over Italian Sara Errani.
On court for three hours and 29 minutes it is the longest recorded match in the season-endning tournament's history.
"After my last match I wasn't that fresh, so I was afraid of what would happen," Radwanska said. "I don't know how I did it today, especially after almost three and a half hours on the court, but I'm so happy.
"Obviously Serena's a great champion -- in that kind of match I have nothing to lose, so I'll just go on court and enjoy it. I've had a lot of tennis the last few days and it's really hard to play like this every day."