Andy Murray refused to blame a wrist injury for his shock defeat at the US Open.

Britain's world number two crashed out at the hands of 16th seed Marin Cilic of Croatia, his first defeat to the 6ft 6ins 20-year-old in their four meetings.

Murray, 22, admitted he had put in a flat performance in going down 7-5 6-2 6-2 at Arthur Ashe Stadium, where 12 months ago he had reached his first grand slam final.

Yet he said his left wrist, which he revealed had been bothering him for more than a week, was not the cause of the surprise defeat in New York.

"Regardless of my wrist, I lost the match," Murray said. "I returned poorly and he served well that was really the difference.

"I felt in the first set, when I was getting into the rallies I was able to move him around and have my chances. And then once he got the first set, he started serving well and started playing really aggressively.

"He was shanking a lot of forehands in the first set, then in the second he started dropping them in and then he started hitting the ball very clean after that and dictated all of the points."

Murray was at a loss to explain why he had not found a way to get back on track after losing the first two sets, having had two set points at 5-4 in the opener.

"I don't know," he said, "that's why I'm disappointed.

"I just didn't find a way to get myself into the match. There were very few long rallies after the first set, and normally I'm able to get myself into rallies.

"I guess on the return, every time I had a chance he would hit a big serve or I would hit a poor return, especially on the second serve; just allowed him to dictate the play."

Perhaps the biggest surprise was the loss of form on his return of serve, particularly as in the previous round against American Taylor Dent, Murray had put on a masterclass in the art.

"Normally the return is the one part of my game where, even if the rest of my game is struggling, I find ways to break serve and get into points, and I didn't do that," he said.

"The first set, I felt like I had the tactics right. I was playing the right sort of way and had the opportunities. Then he started to play a little bit more aggressive and I guess relaxed a little bit after he got that first set.

"You know, it was probably the cleanest I've seen him hit the ball for a set and a half, so that was that was a little bit different to the previous matches.

"But some of that can also be down to being tactically not quite as sound those previous matches."

Asked if the defeat was the biggest disappointment of his career, Murray replied: "My tennis career, yeah. I mean, worse things have happened to me, that's for sure.

"In terms of tennis I guess it's just kind of the way the match went that really makes it disappointing.

"But I believe that I'll come back better from it. I'll learn a lot from what happened this week, like I have done most times when I've had bad results.

"I'll come back better and stronger."