SINGAPORE - No.8 seed Kiki Bertens was a set from securing her second win of round robin play at the BNP Paribas WTA Finals Singapore presented by SC Global when reigning US Open champion Naomi Osaka was forced to retire, trailing 6-3, with an injury.

Read more: 'I deserve to be here' - Bertens ready for Osaka battle in Singapore

The result puts Bertens through to the semifinals after 47 minutes at the Singapore Indoor Stadium.

"When she called out the physio I think the last point she was not moving to the ball like from 4-3 to 5-3. But before that, I didn't know [she was injured]," Bertens said in her post-match press conference.

"I saw her in the match before that she was struggling a little bit with the hamstring, and of course she was taped today, but I think in the beginning she was still okay. It's never easy or never nice to win a match like this, but it is what it is. I'll take it and I'll take my rest, and I have some more recovery time for tomorrow."

Bertens and Osaka played once before at the Abierto Mexicano Telcel in 2016, with the Japanese star winning in straight sets. She began on serve in search of her first win of the week following tough losses to Sloane Stephens and Angelique Kerber.

Champion at the Western & Southern Open in Cincinnati, Bertens needed a win to guarantee her spot in the semis, and soon turned the tables on Osaka in the seventh game to secure the first break of the match.

Osaka called coach Sascha Bajin to court and admitted to feeling physically below her best, taking a medical timeout after Bertens consolidated the break.

As the second set was about to get underway, the youngster ended the match in tears, receiving an ovation from a supportive crowd.

The win is Bertens' 12th against a Top 10 player this season - tying her for the second-most in the last five years, since Serena Williams also earned 12 in 2014. Caroline Wozniacki notched 14 Top 10 wins in 2017.

"I think it's just like proof to myself that I can beat anyone. I think that's a really nice feeling," Bertens said. 

"I think before I really had [the feeling] like, 'Oh, those are great players, and I can never beat them.' Now, I'm still thinking those are great players but I have beaten them before, so I can do it, so hopefully I can do it again today. I think it helped me a lot with the confidence."

Bertens will play either Elina Svitolina or Karolina Pliskova in the semifinals on Saturday; her opponent will be determined by the result between Stephens and Kerber.

"Both matches are going to be really tough. I played Karolina a little bit more this year than Svitolina," she said. "Both tough matches. They played really well here, I would say. So I have to play a high level again and just give it all."