Andy Murray lamented his serving but played down talk of disappointment after a quarterfinal loss to Kei Nishikori at the US Open. The World No. 2 suffered a 1-6, 6-4, 4-6, 6-1, 7-5 defeat to sixth seed Nishikori at Flushing Meadows on Wednesday.

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Murray served at 55 per cent and was broken nine times, while he also lost seven straight games during the fourth and fifth sets.

"It was obviously different serving under the roof. I started off the match serving pretty well," he said.

"It obviously slows the conditions down so it becomes easier to return. He started returning a bit better. I didn't serve so well, obviously. I don't think the reason I lost the match was because of my return game. I broke serve enough times. I just didn't hold serve enough. That was the difference."

Murray came into the year's last grand slam as the form player, having backed up Wimbledon success with another Olympics gold medal and reached the final in Cincinnati.

The three-time grand slam champion, however, was remaining upbeat despite his last-eight exit.

Murray said: "I'm not disappointed in a way. Obviously I would have loved to have won, but I have had a good run every match. I would have loved to have gone further, but it wasn't to be today."