Sloane Stephens and Kiki Bertens have 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.

American Stephens stayed calm and saw off an increasingly agitated Angelique Kerber in the winner-takes-all night match on Friday 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 No.2 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.

In the early match, US 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.

The popular Osaka 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.