From that point a Djokovic victory looked inevitable and, after Del Potro wiped out a final-set break at 3-1 in the following game, the Argentine produced a double fault to give Djokovic another chance for a 5-3 lead.
Williams was fined £13,000 for her meltdown during he US Open final defeat to Naomi Osaka, which saw her issued with three code violations, including a game penalty after calling Ramos a "thief" for docking her a point.
After both players exchanged earlier service breaks in the second set, it became key that Djokovic held onto his serve in the eighth game, staving off three break points in the 22-point game that lasted 20 minutes, 32 seconds.
Novak Djokovic won the U.S. Open on Sunday, his 14th Grand Slam championship.
He is now set to rise to third in the ATP rankings behind Rafael Nadal and Roger Federer, the only two men to have claimed more slam titles, but can justifiably claim to be the best player in the world at present. The 31-year-old Serb has never been considered a once-in-a-generation talent, as have Roger Federer and Rafael Nadal, the ones above him in the trophy count.
Del Potro was the sentimental favourite as he attempted to win a second slam title nine years after his first, having suffered two serious wrist injuries, the second of which he feared would end his career.
"If you told me in February this year when I got the surgery that I'll win Wimbledon, US Open, and Cincinnati, it would be hard to believe", said Djokovic, referring to August's Cincinnati Masters that completed his Masters collection.
"He knows that he's one of my friends on tour and he knows he's one player I want to watch win on tour". "To be honest, I was crying till now", Del Potro said as he began his post-match press conference.
In one charge at the net, Djokovic picked a del Potro volley off his toes and sent back a victor.
Amid increasing noise between points and serves, which prompted frequent interventions from chair umpire Alison Hughes, the previously assured Djokovic - who missed the tournament a year ago due to an elbow injury - appeared agitated.
Federer lost in the fourth round in NY, while Nadal retired from his semifinal against del Potro because of a bad right knee.
Novak Djokovic of Serbia celebrates with his wife Jelena Djokovic after winning his men's Singles finals match. He made one last stand by breaking and holding for 3-all.
In the tiebreak, Del Potro looked susceptible to forehand errors and Djokovic showed enormous courage and quality to take it away from Del Potro, going up 2-0 in the final. Ahead 40-0 he wasn't able to capitalize on any of the three game points, then went onto make two forehand errors to surrender his serve.
You are only a first-time major singles champion once, of course. "But I'm happy for Novak and his team".