Kei Nishikori conceded his five-set battle with Andy Murray took its toll in his US Open semi-final defeat to Stan Wawrinka on Friday.

Wawrinka recovered from a slow start to secure a 4-6 7-5 6-4 6-2 triumph against the Japanese sixth seed Nishikori in New York. 

The loss was a tough one to take for Nishikori, with the 2014 finalist at Flushing Meadows having overcome Murray in an epic to reach the last four.

However, he was unable to replicate that excellent performance against Wawrinka, who will now face Novak Djokovic in the final.

"I was definitely tired, especially in the end," Nishikori said. "The last two sets were really tough.

"I think also he was stepping up a little more from the second set on. He was missing a lot the first set and also I was playing good tennis, but I think he made some changes and he was starting to play better.

"He started using his backhand a little bit more down the line and with angles. I think that was really working for him.

"For me, I think I was hitting too short and wasn't making enough first serves. There is a reason he's been top five for a long time now. He played some good tennis today.

"It's not easy when you have to play number two, number three, and, you know, Novak Djokovic was waiting in the finals.

"It wasn't easy but if I could have won the last few matches a little easier I could have had more of a chance today and maybe reach the final."