(Reuters) - Australian Nick Kyrgios pulled out of his first-round match with Britain's Andy Murray at the Winston-Salem Open on Sunday due to knee pain but the 26-year-old is confident he will be fit for next week's U.S. Open Grand Slam.

Kyrgios withdrew from the ATP 250 event in North Carolina in a last-minute call, with lucky loser Noah Rubin replacing him. American Rubin lost 6-2 6-0 to former world number one Murray.