Working his way back from elbow surgery he underwent following the Australian Open, Djokovic did not expect to feature in the Grand Slam picture this season.
But in the last eight weeks the 31-year-old has resembled the Grand Slam machine that held all four major titles at the same time in 2016, wining three of his last four events including Wimbledon and U.S. Open.
"If you told me in February this year when I got the surgery that I'll win Wimbledon, U.S. Open, and Cincinnati, would be hard to believe," said Djokovic, after pushing his career total to 14 Grand Slams to sit equal third with Pete Sampras on the all-time list.
"Life showed me that it takes time for good things, it takes time to really build them, for things to fall into place.
"The last two months have been terrific."
The sixth seed had suffered in the ferocious heat and humidity through the early days of the season's final Grand Slam but dropped only two sets for the tournament and none since the second round.
Playing on the same court where Sampras claimed his 14th and final Grand Slam crown, Djokovic has endured more disappointment than joy on Arthur Ashe, winning three titles from eight finals.
All that frustration was washed away when he struck the overhead winner to clinch victory over Del Potro, Djokovic falling to his back, arms and legs spread savouring the moment.
"Pete Sampras is one of the biggest legends ever to play the game," explained Djokovic. "He was my childhood idol.
"There is a lot of significance of me being now shoulder to shoulder in terms of Grand Slam wins with him.
"It's truly incredible when you think about it. I watched him win one of his first Wimbledon championships, and I grew up playing and thinking that one day I'll be able to do what he does.
"To actually be here, it's a dream come true."
Djokovic and Del Potro walked onto Arthur Ashe Stadium court with the Billie Jean King National Tennis Center still buzzing over the controversial women's final on Saturday, won by Japan's Naomi Osaka after Serena Williams incurred three code violations.
Britain's Alison Hughes, the second woman to umpire a U.S. Open men's singles final, kept a low profile as the two former champions went to work.
The tension and fireworks of the Osaka-Williams clash were left behind during a relatively calm and composed baseline battle between two friends and familiar foes, who had played each other 18 times, with Djokovic holding a commanding 14-4 edge.
While there was joy in victory there was none to be taken from beating a teary Del Potro, who knows the pain of a comeback having undergone four wrist surgeries that caused him to miss 14 Grand Slams and at one point looked to be derailing his career.
Del Potro's sole Grand Slam title came at the U.S. Open in 2009 and he said it had been an emotional journey back to the Flushing Meadows final.
"I'm very sad for being a loser today but Novak deserved to take the trophy," said Del Potro, who wept into his hands before being comforted by Djokovic.
"I was playing almost at the limit all the time, looking for winners with my forehands, backhands, and I couldn't make it because Novak were there every time.
"The worst part to me is the chances that I couldn't make but then when you see a friend holding the trophy, it's good, too.
"I'm glad that Novak is the champion."
(Editing by Ian Ransom/Peter Rutherford)
