With the win, top seed Sinner claimed his second Grand Slam title after winning the Australian Open earlier this year.
Sinner was under a cloud of controversy at the start of the tournament after revelations that he tested positive twice for an anabolic agent in March but avoided a ban when an independent tribunal accepted his claim that the positive tests were the result of an unintentional contamination.
The top seed blocked out the furore in New York and with the win claimed his second Grand Slam title after winning the Australian Open earlier this year.
"So many big wins for me this season, starting off with Australia and playing so well there which gave me confidence for until now," said Sinner.
Fritz got off to a dreadful start when he handed Sinner the break with an unforced error in the first game but settled his nerves, breaking back in the fourth game and surviving a 23-shot rally to save break point in the fifth.
But it was only a matter of time before the Italian would hit his stride and he broke the 12th-seeded American with a perfectly placed drop shot in the seventh game. Fritz dropped his serve again on set point.
The fired-up home crowd tried to lift Fritz with chants of "USA!" in the second set.
However, the momentum swung in Sinner’s direction as he broke Fritz on set point from the baseline and he piled further misery on the American when he clawed back from triple break point down in the opening game of the third set.
Fritz clung on and even went up a break but was left smacking his racket onto the court in frustration when he hit a volley into the net to let Sinner break back in the 10th.
Sinner broke for a sixth time in the contest to wrap up the win, with Fritz swiping a forehand into the net on match point.
With U.S. great Andre Agassi on court for the presentation ceremony, Fritz told the crowd: "I know we’ve been waiting for a champion for a long time so I’m sorry I couldn’t get it done this time.
"But I’m gonna keep working and hopefully I’ll get it the next time."
