Novak Djokovic admitted Sunday that his nerve failed him as he slipped to a 6-7 (1/7), 6-4, 7-5, 6-3 defeat to Stan Wawrinka in the US Open final.
The 29-year-old world number one, who was chasing a third New York title and 13th career major, suffered his ninth loss in 21 Grand Slam finals.
In a final which lasted almost four hours, Djokovic converted just three of 17 break points.
"I lost my nerve in the important moments. He kept his cool," said the 2011 and 2015 champion.
"I think that's what decided the match. 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."
The Serb star also admitted that he came close to skipping the tournament as a serious wrist injury threatened to compromise his assault on a third major title of the year.
Physical problems plagued the world number one throughout the two weeks, a right shoulder injury giving way to problems with his toes as his hopes of a third US Open slipped away.
"Some other injury was very serious at the time. I really didn't know whether or not I'm going to come, to be honest," said the 29-year-old Djokovic who ended the Grand Slam season with a sixth Australian Open and a maiden Roland Garros.
Djokovic caused controversy at 1-3 and 2-5 down in the fourth set when he was allowed a medical timeout for treatment on his feet.
Wawrinka was angry that Djokovic had been granted permission to call the trainer and doctor while he was about to serve.
- Serve not there -
Djokovic apologised to his opponent as he sat courtside, claiming he couldn't stand as a consequence.
"It was just the toenails were off and bleeding. Yeah, it was quite painful to move around," he said later.
But he insisted that he was not trying to gain an advantage by forcing Wawrinka to wait to serve at a crucial part of the final.
"Why would I be surprised if I was allowed?," he snapped at a reporter, adding that it was medical necessity.
Djokovic refused to pin the reason for his loss on the toe injury but he did admit that regular difficulties with his right arm did little to help his cause.
"I made a lot of double faults throughout this tournament. I was struggling with the motion, with the movement and on the serve, with the technique.
"I was working a lot on it and trying to find that rhythm, but my body has kind of compensated and made some different things to protect the problem I had with the arm.
"Unfortunately the serve wasn't there when it was needed."
Djokovic refused to get too downcast with his season despite a shock third round loss at Wimbledon.
His French Open victory allowed him to complete a career Grand Slam while victory in Paris also meant he held all four Slams at the same time.
"Winning four Grand Slams in a row for me was an incredible achievement. I'm very proud of it, so this loss today cannot overshadow the great moments I have had in Australia and especially in Paris," he said.
The world number one hailed Wawrinka who also beat him in the French Open final last year, also having lost the first set.
"He loves to play in the big matches. He comes up with his best game. He's a very complete player. Sometimes if he feels right he doesn't miss much and makes a lot of winners and it's hard to play him. That's what happened today."