Bertens into WTA last four, Osaka retires

Kiki Bertens has qualified for the semi-finals of the season-ending WTA Finals with a tearful Naomi Osaka retiring after the first set of their group match.

Japan's Osaka retired on Friday with what appeared to be a leg injury after dropping serve at love to hand the Dutchwoman the set 6-3. Osaka, the US Open champion, had a medical time-out after the previous game but was finally unable to continue.

Bertens, who won the last four games from 3-2 against her ailing opponent, is assured of a place in the last four because, under WTA tournament rules, the match counts as a straight-sets win for her.

She finished with a 2-1 record to advance with the winner of the final group match later on Friday between top-seeded Wimbledon champion Angelique Kerber of Germany and American Sloane Stephens.

"It's never nice to win a match like this and I hope she's feeling better soon," the 26-year-old Bertens said in a post-match interview.

"I will just try to keep on going. I have had some tough matches here already this week but the day in between helps for recovery."

Bertens is only in the Singapore tournament after top seed Simona Halep's withdrawal last week through injury.

Ukraine's Elina Svitolina and Czech Karolina Pliskova on Thursday reached the last four of the $US7 million ($A9.9 million) tournament from the white group.

Reuters