Kei Nishikori's Wimbledon preparations have been hit by injury as he was forced to retire from his Gerry Weber Open clash with Karen Khachanov.

The Japanese was playing in his first grass-court tournament of the year in Halle but was trailing 3-2 in the round-two contest when he pulled out of the match with a back problem after just 34 minutes.

It marks further injury concern for Nishikori who was plagued by a wrist injury for much of the clay-court season and, although he reached the quarter-finals at the French Open, he has not won a title in 2017.

Nishikori has never gone beyond the fourth round at Wimbledon, which starts in 11 days' time.