Singapore - A confident Sloane Stephens stormed into a semi slot at the WTA Finals with a convincing straight sets victory against top seed Angelique Kerber on Friday, joining fellow debutant Kiki Bertens in the knockout stage.
The former US Open champion continued her unbeaten run in Singapore with a 6-3, 6-3 triumph in 101 minutes, her fifth straight-sets victory in a row over Kerber.
After topping Red Group, Stephens will play Czech Karolina Pliskova in the semi-finals.
For the first time since 2003, none of the top four seeds advanced to the semi-finals of the WTA Finals.
"I stayed solid the whole time knowing there was a lot on the line," Stephens said after the match.
"I knew it was going to be a battle and just tried to play as hard as I could."
The match started tensely, with Kerber unable to convert seven break points in Stephens' first three service games, and the missed opportunities proved costly.
An energetic Stephens made her pay by converting all three of her break points to draw first blood.
The American impressively counterattacked and pinpointed the lines to frustrate Kerber, who survived an injury scare when she landed awkwardly midway through the first set.
Kerber was immediately under pressure in the second set and saved seven break points in the fourth game before cracking under the pressure.
There was still fight left in the German, who broke straight back but continued to struggle on serve.
On her fifth break point, Stephens claimed the decisive break in the eighth game and then served it out to knockout the Wimbledon champion.
Bertens faces unbeaten Elina Svitolina after a hamstring injury forced Naomi Osaka's retirement earlier on Friday.
She won the first set 6-3 in 47 minutes before US Open champion Osaka, who wiped away tears, forfeited the match.
It was a bitterly disappointing end for the 21-year-old, who came into Singapore as the form player but lost three-set matches to Stephens and Kerber.
"I hurt my left hamstring in the first match (against Stephens)," Osaka told reporters.
"It was just getting worse and worse every time I played. I couldn't even really practice that well today."
But it was enough for Bertens to seal a final four spot having defeated Kerber earlier in the round-robin phase.
Bertens only qualified for the WTA Finals just days before the tournament after world number one Simona Halep withdrew due to injury.
"It's never easy or never nice to win a match like this, but it is what it is," Bertens said.
"To be at the last four, last tournament of the year, it's unbelievable."
Osaka had started the match brightly, rolling through her service games before being broken in the eighth game.
With her upper leg heavily strapped, Osaka called a medical timeout after the eighth game and required treatment in the locker room.
She came back on court but dropped her serve to love, culminating in a double fault which turned out to be her last action at the WTA Finals.
Results from the WTA Finals tennis in Singapore on Friday (x denotes seeding):
Sloane Stephens (USA x5) bt Angelique Kerber (GER x1) 6-3, 6-3
Kiki Bertens (NED x8) bt Naomi Osaka (JPN x3) 6-3, Osaka retired hurt