Novak Djokovic was unwilling to make excuses as he lamented missed opportunities in his US Open final loss to Stan Wawrinka.

The world number one suffered a 6-7 (1-7) 6-4 7-5 6-3 loss to Swiss third seed Wawrinka in the decider at Flushing Meadows on Sunday.

Djokovic needed treatment during the fourth set, revealing his "toe nails were off and bleeding".

Despite that, the 12-time grand-slam champion played down the impact and said his poor break-point conversion (three of 17 won) was costly.

"I just didn't capitalise at all on my opportunities. I had plenty of them, break points. It was a terrible conversion of the break points. Just terrible from my side," Djokovic said.

"In the matches like this, if you don't use the opportunities, the other guy comes and takes it. And that's what he did.

"That's why I said he was more courageous, because he stepped in and played aggressive where I was kind of more waiting for things to happen. And that's it."

The third set proved particularly costly for Djokovic, who converted just one of his six break points as Wawrinka took both of his.

The 29-year-old also lamented his inability to stay focused in the big moments.

"I lost my nerves in the important moments. He kept his cool. I think that's what decided the match," Djokovic said.

"I guess sometimes it happens, even though you have the experience and know what to do.

"Just the heat of the moment and importance of the match, I guess, you know, was too strong for me at certain periods of the match. Just if you lose your cool, the match can go away."