The Argentine frustrated by his four-set loss to the world No 1 after winning the first set of semi-final clash in New York.
Juan Martin del Potro backs Rafael Nadal to go on to become US Open champion
Juan Martin del Potro admitted he was "angry" over his US Open semi-final loss to Rafael Nadal before backing the Spaniard to claim a third New York title and 16th grand slam triumph.
Del Potro dominated the first set as he sought to defeat Nadal just as he did at the same stage of the tournament in 2009 before going on to beat Roger Federer in the final for his only title at the majors.
But the emotional and physical toll of seeing off Federer again in the quarter-finals this year, and having to come back from two sets down and save two match points against Dominic Thiem in the last 16, proved too much of a burden.
"To be honest, I'm angry to lose a chance like this, but maybe tomorrow, after tomorrow, I will be calm and see how big the tournament was for me," said Del Potro after his 4-6, 6-0, 6-3, 6-2 loss to the world No 1.
He broke the Nadal serve only once - in the fifth game of the opening set - and even that was off a fortunate net cord.
"I think I never had control of the match. I was lucky to break his serve with the net point," he admitted.
"When you don't have that confidence to play three, four hours with a good backhand against Rafa, is just a matter of time to get down your game.
"But also he improved very much his game after the second set, and his balls came too fast from both sides. He deserved to win, for sure."
Argentine 24th seed Del Potro was playing in only his fourth major semi-final and first since 2013, when he lost a five-set Wimbledon epic to Novak Djokovic.
Friday's loss was his ninth in 14 meetings against Nadal and he was quick to tip his conqueror to defeat Kevin Anderson in Sunday's final and add to his 2010 and 2013 titles in New York.
"He had the game today to win, to beat me, and to win the tournament," was the 28-year-old's prediction.
"I think he's playing much better. He's improved his backhand and he serves good. He's the number one of the world. He's winning many tournaments this year."
Del Potro has long been a sentimental favorite on the tour, a status created by his lengthy battles against wrist injuries, which required four surgeries.
When his world ranking, once as high as four, slumped to 1,045 last year, he even pondered retirement.
"I was exhausted after the Thiem match, the Roger match. I had flu during the week," he added.
"So I had many problems before this match, but I was very motivated to play the semi-final. I gave all my effort to survive this match, but I couldn't do well."