Madison Keys retires in Japan Women's Open quarter-finals

Kumkhum was leading 6-2 4-3 when Keys - who saved eight break points at the start the second set - decided to withdraw from the WTA International hard-court tournament.

The American second seed played with heavy strapping on her right shoulder, having had the trainer work on that area in her second-round match with Misaki Doi on Wednesday, but could not complete the contest, retiring in visible distress.

Keys' season is now over, but the 19-year-old was quick to stress the positives.

She posted on her Twitter account: "Not how I wanted to end the season but still a good year and now back home to rest up!"

Thailand's Kumkhum will now face fifth seed Zarina Diyas in the last four after the Kazakhstani came from a set down to beat Ana Konjuh of Croatia 6-7 (4-7) 6-4 6-2.

Konjuh squandered a 5-2 lead in the first set but recovered to win the tie-break. However, world number 37 Diyas levelled to force a decider, which she wrapped up confidently after racing into a 4-0 lead.

The winner of the semi-final between Kumkhum and Diyas will compete in their first WTA final.

Whoever progresses is likely to face a stern test, with top seed Samantha Stosur displaying ominous form in Osaka.

Australian Stosur needed just an hour and eight minutes to reach the last four by eliminating Diyas' compatriot Yulia Putintseva, sending down nine aces in a 6-4 6-1 success.

Standing in the way of Stosur will be Ukraine's Elina Svitolina, the third seed having overcome American eighth seed Lauren Davis 6-2 6-4.

Svitolina lost just 12 points on serve in a straightforward victory, keeping her hopes of a third WTA title alive.