Azarenka defeats Li Na to secure year-end world No. 1 ranking

(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.

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"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.

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"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."