Osaka
Naomi Osaka. Source: Yahoo.

Reigning US Open champion Naomi Osaka was reduced to tears as she was forced to retire from the WTA Finals against Kiki Bertens, handing the Dutchwoman a surprise spot in the semi-finals.

Bertens won the first set 6-3 in 47 minutes at the Singapore Indoor Stadium before Japanese sensation Osaka, who wiped away tears, forfeited the match.

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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 Sloane Stephens and Angelique Kerber.

But it was enough for Bertens to seal a final four spot in her debut at the season finale, having defeated top seed 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 nice to win a match like this,” Bertens said after the match. “I want to try to keep on going. I’ve had a lot of tough matches here.”

Osaka, who was playing in her first WTA Finals 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 the court but dropped her serve to love, culminating in a double fault which turned out to be her last action at the WTA Finals.

Kerber plays the unbeaten Stephens in the last round-robin match from Red Group, with the winner joining Bertens in the semi-finals.