Saturday's semi-finals will see Bertens take on White Group winner Elina Svitolina of Ukraine, before Stephens faces big-hitting Czech Karolina Pliskova in the night match of a tournament that has seen the four lowest-ranked players advance
Sloane Stephens and Kiki Bertens continued their dream debuts at the WTA Finals when the tournament rookies won their final Red Group matches to advance to the last four of the season-ending event in Singapore on Friday.
American Stephens stayed calm and saw off an increasingly agitated Angelique Kerber in the winner-takes-all night match to complete round-robin play with a spotless 3-0 record after she wrapped up a 6-3 6-3 victory against the German.
In the afternoon match, Bertens advanced to the semi-finals after Naomi Osaka retired with a left hamstring injury at the end of a first set the versatile Dutchwoman had captured by a 6-3 margin.
Kerber enjoyed a brilliant season with her third Grand Slam title at Wimbledon in July but the world number two seems to struggle in Singapore and was once again undone by Stephens, who now boasts a 5-1 head-to-head record against the German.
"I stayed solid the whole time. I knew it was going to be a battle against one of the toughest players on the tour, so I just came out and played as hard as I could and am really pleased with the win," Stephens said in a post-match interview.
Stephens staved off seven break points early in the first set and added to the German's frustration by converting the first two she fashioned to pull clear for a 5-2 lead after little could separate the baseliners in the opening exchanges.
An ace brought up the first of three set points but she was unable to convert any of them as Kerber made it eighth time lucky to pull a game back. Her joy was short-lived as Stephens broke again to seal the opener.
The games were getting longer as the intensity levels increased and the pair traded breaks early in the second set until Kerber's form dipped when she was broken in the eighth game and the American gleefully raced away to seal victory.
In the early match, U.S. Open champion Osaka took to the court with tape on her left leg for a problem that flared up in her loss to Stephens on Monday, and was competitive for half a set before opting to withdraw after Bertens took the opener 6-3.
Perhaps mindful of the injury, Osaka went for her shots from the outset, a tactic that comes with the risk of giving away free points, while Bertens was content to keep the ball in play and pin the powerful Japanese player to the back of the court.
Both strategies worked in the early stages until Osaka pushed the limits a little too far in the seventh game and was broken after she sent a string of forehands either long or wide.
Osaka left the arena for a medical timeout after Bertens held easily to move 5-3 ahead. The 21-year-old Japanese returned with her thigh heavily strapped and was immediately broken to love as she hobbled painfully around the baseline.
The popular Osaka called for the trainer once more but tearfully succumbed to her fate after consulting with her coach to end her breakthrough season with a disappointing three straight defeats at the year-end championships.
Saturday's semi-finals will see Bertens take on White Group winner Elina Svitolina of Ukraine, before Stephens faces big-hitting Czech Karolina Pliskova in the night match of a tournament that has seen the four lowest-ranked players advance.