Azarenka defeated eighth-seeded Li Na 7-6 (7-4), 6-3 in the final match of the round-robin section of the tournament and grabbed the second semifinal slot for the Red Group. She'll go against White Group winner Maria Sharapova in the semifinals Saturday.
Azarenka entered the tournament needing two wins to secure the No. 1 ranking. She lost to Serena Williams in group play but then beat Angelique Kerber and Li to make sure she will end the year with the No. 1 ranking.
Azarenka finished with six breaks against Li. Each dropped serve twice in the seesaw first set.
Sharapova, seeded second, completed a 3-0 run through the White Group with a 6-0, 6-3 rout of Samantha Stosur in 65 minutes. Sharapova dropped nine points over seven games serving and saved the only break point she faced. She took 58 percent of the points off Stosur's serve.
Fourth-seeded Agnieszka Radwanska advanced, surviving a 3 1/2-hour battle against No. 7-seeded Sara Errani. Radwanska lost her serve eight times but ended up with 10 breaks, including four in the final set.
It was the second consecutive match of more than 3 hours for Radwanska, who next faces Williams, seeded third. Williams, who had Friday off, is 3-0 against Radwanska, including a three-set win in the finals at Wimbledon this year.
Saturday's meeting will be the sixth this year between Azarenka and Sharapova. Four of them, including at the Australian Open, were tournament finals. Azarenka is 4-1 in 2012 and 7-4 all-time against Sharapova.
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