Japan's Kei Nishikori has pulled out of next week's ATP 250 tournament in Cincinnati to rest a minor hip injury ahead of the US Open.

The world number four, who reached the final at Flushing Meadows last year, looked out of sorts in collapsing to a one-sided 6-3 6-0 semi-final loss to Briton Andy Murray in Montreal on Saturday.

The 25-year-old hard-court specialist, the first Japanese man to reach a grand slam final, said he did not think the injury would curtail his title bid at the US Open which starts in New York on Aug. 31.

"I've never injured that part of the body but I expect to get better in a week," Nishikori said in a statement carried by Kyodo News on Monday.

"I think I'll be able to play well at the US Open."