Fritz beat Zverev at Wimbledon in an epic five-setter and there was little to separate them in the opening set in New York when they traded blows from the baseline all the way to a tiebreak, where Zverev floundered with his forehand and the American took the set with an overhead smash at the net.
Zverev bounced back with a different racquet and broke at 4-3 in the second before serving out the set to level the match, but Fritz returned the favour early in the third when he converted his first break point before racing into a 3-0 lead.
Although Zverev clawed his way back, he was once again forced to save set points after some nervous errors before Fritz finally took the third set when the German found the net as the crowd erupted.
With no breaks on offer in the fourth, Fritz got the upper hand in the tiebreak as Zverev once again faltered on crucial points before the American notched one of the biggest wins of his career.
"I did nothing to deserve to win. It's as simple as that. I played terrible," an angry Zverev said afterwards, adding that his backhand let him down.
"My most reliable shot, the shot that I'm most known for, the shot that you normally wake me up at 3:00 a.m. and I would not miss, was absolutely not there today and I have no words for it.
"At some point, I didn't know what to do anymore."