UPDATE 1-Tennis-World number one Osaka retires with knee injury

Japanese world number one Naomi Osaka retired from her Cincinnati Masters quarter-final with an apparent knee injury on Friday.

Osaka, who was playing American Sofia Kenin, sought treatment on the knee in the third set, then pulled out. Kenin was ahead 6-4 1-6 2-0 at the time. The injury comes less than two weeks before the start of the U.S. Open, where Osaka is the defending champion.