Fourth seed Angelique Kerber reached the Pan Pacific Open semi-finals in Japan on Friday after American Madison Keys retired due to injury early in the deciding set.
With the match clock at just over two hours, Kerber grabbed an early break to lead in the final set at 6-4 4-6 2-1, before Keys pulled out of the contest.
"It was a really good match in the first two sets, and this is never the way that you want to win the match," three-time Grand Slam winner Kerber said.
"I hope Madison will recover really soon so she is ready for the next tournaments."
After a run to the Eastbourne final in early July, Kerber won just one match at Wimbledon - where she was the holder - and arrived in Osaka on the back of a five-match losing streak.