6-3 victory under the roof at Arthur Ashe stadium - it was raining in NY - pulled him level with Pete Sampras for third on the all-time men's list. He almost quit tennis in 2015, after four wrist surgeries that wouldn't let him regain the form that carried him past Roger Federer to win the 2009 U.S. Open.
Djokovic's strength lay in his returns. We reached the top after three hours.
On the other hand, Del Potro was relying heavily on his serves and power.
Djokovic collects $3.8 million and his third U.S. Open championship.
Yet the atmosphere was more positive on Sunday as spectators enjoyed an absorbing contest - again played under the roof of Arthur Ashe Stadium due to rain - between two great friends. That inspired me to start playing tennis.
Djokovic then got right back to work, and announced that he was, once more, himself by winning Wimbledon.
Throughout the tournament, the two struggled and rose above New York's searing heat and hard humidity. He made one last stand by breaking and holding for 3-all. From there Djokovic comfortably served out the set. Djokovic, up a set, was serving at 3-4 when the two proceeded to play 23 points.
Four easy holds later and it was onto a tiebreaker that decided the second set, and ultimately the course of the match as Djokovic capitalised on several bad errors from his opponent to win it 7-4.
The tournament, meanwhile, set a new attendance record of 828,798 fans through the gates at Flushing Meadows.
The quest to win a second major will have to wait until next year, but del Potro, who turns 30 this month, feels good about his chances with the way his wrist held up over the last two weeks.
Djokovic occasionally seemed annoyed by the support with the noise as British umpire Alison Hughes had to constantly plead with the crowd to remain quiet during points.
Things looked ominous for Del Potro as Djokovic seized control early on, moving magnificently and constructing a number of superb points with precision striking. Twice, the Serb tried to attack at the net. After taking just two points off Del Potro's first three service games, Djokovic pounced to break for 5-3.
Perhaps the biggest loser in it all was the eventual champion, Naomi Osaka, whose moment of glory was taken away in the aftermath of a tirade by Williams.
In the tiebreak, Del Potro looked susceptible to forehand errors and Djokovic showed vast courage and quality to take it away from Del Potro, going up 2-0 in the final. What was different was that the quality of tennis only kept getting better.
All that frustration was washed away when he struck the overhead victor to clinch victory over Del Potro, Djokovic falling to his back, arms and legs spread savouring the moment.
Djokovic sprinted into a 3-1 lead in the third set before a battling Del Potro clung on, hitting back for 3-3.
This was Djokovic's third championship in NY, along with those in 2011 and 2015.