Williams improved to 11-1 lifetime versus Azarenka with a 6-4, 6-4 victory. She has taken the last nine matchs, including the final of this year's U.S. Open.
Williams ended up with five breaks in 10 receiving games, finishing off all but one of her break chances. Azarenka managed three breaks herself, but Williams seemed more solid than in the uncharacteristic off serving games Wednesday.
Williams, seeded third, went 3-0 and didn't drop a set in finishing atop the Red Group standings. Azarenka and eighth-seeded Li Na meet Friday with the other group semifinal berth at stake. They are 1-1 in group play.
Li kept her hopes of advancing alive with a 6-4, 6-3 decision over fifth-seeded Angelique Kerber. Li took 57 percent of the points off Kerber's serve and finished off three breaks in each set. Kerber finished group play 0-3.
Seventh-seeded Sara Errani also earned a meaningful Friday match, beating Samantha Stosur 6-3, 2-6, 6-0 to get to 1-1 in the White Group. Stosur had a solid second set to even the match but Errani dominated the third set, winning two-thirds of the points overall.
Errani will go against fourth-seeded Agnieszka Radwanska Friday with the winner joining No. 2-ranked Maria Sharapova as the White Group semifinalists. Sharapova, 2-0 in the group, plays Stosur (0-1), who is subbing for Petra Kvitova (0-1) after Kvitova withdrew due to illness. Errani and Radwanska are both 1-1 in round-robin play.