ISTANBUL - Victoria Azarenka of Belarus will finish the year as world number one after a 7-6 6-3 win over China's Li Na in her final round robin match at the WTA Championships on Friday.

The Australian Open champion managed the two wins she needed in the round robin competition to end 2012 in top spot by beating Germany's Angelique Kerber on Wednesday and then Li.

Li Na of China in action against Victoria Azarenka of Belarus during day four of the season ending WTA Championships Tennis at the Sinan Erdem Dome in Istanbul, Turkey, on Oct 26, 2012. [Photo/CFP]

Azarenka now faces world number two Maria Sharapova in the semifinals with the Russian, who beat Sam Stosur 6-0 6-3 on Friday, having already sealed first place in Group B before playing the Australian who could not reach the last four.

Poland's fourth seed Agnieszka Radwanska will play Serena Williams in the semis after overcoming seventh-seeded Italian Sara Errani 6-7 7-5 6-4 in the longest match ever played at a WTA Championships which lasted three hours, 29 minutes.

Azarenka becomes the 11th year-end number since the WTA began its rankings in 1975 and the fifth this century.

"It's kind of difficult to believe that a little girl from Belarus is on that list," the 23-year-old said. "But it's really an incredible achievement.

"When I started to play tennis I had this big picture in my head that I want to be there. Back then it was so far away. It was like pretty much touching the sky," added Azarenka.

The Belarussian broke Li when the eighth-ranked Chinese served for the first set at 5-4 and was forceful in the tiebreak, winning it 7-4.

Azarenka dominated the action off the ground for much of the second set and sealed the match after forcing Li into a forehand error in a hard hitting rally.