Andy Murray 'proud' despite US Open loss to Kei Nishikori
Britain's Andy Murray said he had "not let anyone down" after losing to Japan's Kei Nishikori in the US Open quarter-finals.
Sixth seed Nishikori won 1-6 6-4 4-6 6-1 7-5 to inflict an earliest Grand Slam defeat of the season on Murray.
The Scot, 29, finished runner-up at the Australian and French Opens before winning Wimbledon and Olympic gold.
"I tried my best. I fought as hard as I could with what I had today," said the world number two.
"I pushed myself as hard as I could over the last few months and I'm very proud of how I've done."
The defeat was only the second in 28 matches for Murray, who had reached the final of his previous seven tournaments, stretching back to the Madrid Open in May.
That run brought him his first Italian Open, fifth Queen's Club, second Wimbledon and second Olympic titles.
"If someone had offered me the summer that I have had before Wimbledon, I probably would have signed for that," said Murray.
"After a few days away, I would imagine I'd be very happy with how I've done and learn from this match and the summer as a whole, because it's been tough.
"I'm happy how it's gone. There's just a few things I could do differently next time."
Murray let a two-sets-to-one lead slip against Nishikori and became embroiled in an argument with the umpire and tournament referee after play was halted when the stadium sound system emitted a loud noise, with the Scot holding break point.
"They stopped the point and I was just curious why that was and that was it," said Murray, who went on to lose seven games in a row.
"Did it affect me? Definitely I would say to 4-1.
"I didn't play a good game after I got out of the change of ends and then he held pretty comfortably the next game. But after that I don't think so."
Murray's form dipped after an impressive first set, with conditions changing when rain prompted the roof to be closed in the second set.
The roof remained shut despite the weather improving at Flushing Meadows.
"We were told at the beginning of the event and also today that if the forecast's good they will open the roof during the match if that's the case," said Murray.
"Why that didn't happen today I don't know. It's not really for me to say."
Murray will next head to Glasgow for Britain's Davis Cup semi-final against Argentina, which begins on 16 September.
BBC tennis correspondent Russell Fuller:
"Early in the fourth set, with Murray leading by two sets to one and a break point up - and in control of the rally - a let was called as a sound like a gong was heard under the closed roof. Murray allowed himself to be distracted for too long as Nishikori ran away with the set to take the match the distance.
"A hugely successful summer started to take its toll on Murray here in New York. But having already won Wimbledon and Olympic gold, this may be a disappointment that is not that hard to swallow."