By Matt Cronin
ISTANBUL (Reuters) - 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.
Azarenka now faces world number two Maria Sharapova in the semi-finals 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.
Radwanska next faces 15-times grand slam champion Williams, who has yet to drop a set at the tournament.
Radwanska, who had lost to Sharapova in a three-hour and 12 minute marathon on Wednesday, had to pull out all the stops to get past the determined Errani.
The Italian won the first set 8-6 in the tiebreak after coming out ahead in a 34-ball rally and pumped her fist in delight while Radwanska angrily threw her racket to the ground.
Errani fought off four break points to hold for a 5-4 lead in the second but Radwanska responded brilliantly, winning 12 of the next 13 points to take the set 7-5.
"When I'd played for so long already I was thinking, just pretty much fight until the end," Radwanska said.
Errani troubled the Pole with a mix of heavy topspin and relentless retrieving but was unable to stay as mentally focused as the 2012 Wimbledon finalist, who nearly squandered a 3-0 lead in the third set but finally managed to defeat the Italian.
"She's a big fighter so she didn't give up," Radwanska said. "So I really had to play my best tennis until the end of the match."
(Editing by Ken Ferris)