Elina Svitolina became the latest seed in the women's draw to make an exit after being well beaten by unseeded Garbine Muguruza in the third round.
Ashleigh Barty claimed a record-setting US$4.42 million paycheque - the largest sum ever awarded in men's and women's tennis.
Ashleigh Barty, playing in her WTA Finals debut, will look to cap off a momentous season when she plays Elina Svitolina in Sunday's final.
World No.12 Sofia Kenin replaced Bianca Andreescu after the US Open champion suffered a knee injury against Karolina Pliskova on Wednesday.
Simona Halep still has a chance of qualifying having overcome US Open champion Bianca Andreescu in a three-set thriller in her opener.
WTA Finals: Simona Halep moved on top of Purple Group alongside defending champion Elina Svitolina who earlier beat No.2 seed Karolina Pliskova.
US Open: Serena Williams, who is seeking a 24th Grand Slam singles title to match Margaret Court's all-time record, brushed aside fifth seed Elina Svitolina 6-3, 6-1.
Serena Williams dismantled Wang Qiang 6-1, 6-0 in a complete mismatch that was the quickest at this year's US Open to set up a semi-final clash with Ukrainian trailblazer Elina Svitolina.
Roger Federer continues his first French Open campaign in four years on Friday with a third-round match against Norway's Casper Ruud, while reigning champion Rafael Nadal faces Belgian David Goffin.
Alexander Zverev extended his terrible clay-court run, falling 7-5, 7-5 to Italian wild card Matteo Berrettini in the second round.
Naomi Osaka emerged from an error-strewn first set against crafty Latvian Anastasija Sevastova to win 4-6, 6-3, 6-4.
Naomi Osaka defeated Lativia's Anastasija Sevastova to reach the Brisbane International semi-finals.
Svitolina looked sluggish after playing the two longest matches in the tournament against Wozniacki and Bertens in the semi-finals.
Svitolina has looked on a mission at the WTA Finals in an effort to make amends for a lackluster early exit last year.
Wang Qiang wrapped up the biggest win of her season on Saturday after an overnight rain delay forced her to wait to claim victory over Elina Svitolina.
Elina Svitolina, the world number four, has now successfully defended three career titles, in Baku, Dubai and also Rome.
Elina Svitolina will be bidding to win her eighth successive final dating back to last season as she bids for a third title this year.
Angelique Kerber and Elina Svitolina will face off in a Dubai Championships semi-final on Friday after both ran down opponents in quick straight-sets wins.