Bertens was a late alternative for world No.Three Naomi Osaka, who earlier within the day withdrew from the round-robin match as a outcome of a shoulder damage.
The Dutchwoman replaced Naomi Osaka Tuesday after a back injury ended the Japanese superstar's season, and she duly produced a stirring comeback to oust the world's top-ranked player Ashleigh Barty 3-6 6-3 6-4 in the Red Group.
The Australian was disappointed to lose against Kiki Bertens on Tuesday from a strong position but made no mistake this time, surging into a 4-0 lead in the second set and saving all four break points she faced.
The world No.10 gained Three-6, 6-Three, 6-Four in two hours and 9 minutes in a see-saw match that includes 12 breaks of serve.
"(Losing at Wimbledon) was the lowest emotional feeling I've ever felt", she said. Exploiting her opponent weakness, Barty pounced to run away with the first set. Bertens is 1-0 and Barty 1-1 in the Red Group standings. The pair recently played in Beijing with Barty topping the Czech seasoned campaigner "I've played Petra a number of times this year".
"I just want to train really hard", Osaka said. It's just a little bit painful.
Bertens faces Belinda Bencic next on Thursday night in her bold quest to become the first alternate to make the semi-finals at the season-ending championships.
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The Australian world number one, 23, will earn at least $1.45million regardless of whether she wins Sunday's WTA Finals decider.
World No.1 Ashleigh Barty doesn't only sit on top of the WTA Rankings but also on top of the WTA Prize Money Charts for 2019.
Bertens received medical attention after the opening game of the second set, when her blood pressure and pulse were checked, before becoming the second player in the tournament to retire mid-match after Bianca Andreescu withdrew against Karolina Pliskova on Wednesday.
"It kind of sucks because I thought I was playing well".
Kvitova, the oldest player in the draw, changed tactics early in the second set and attacked the net but the French Open champion had the answers to reach the semi-finals on her WTA Finals debut. "It was really hard to bend with the taping and the pain kept getting worse, so I had to stop", she added.
It was just two years ago that Bertens considered quitting tennis even with all the success she was enjoying.
"I think definitely she was dictating the game today".