He overcame a 95-minute set, the longest of the tournament, to beat Juan Martin del Potro 6-3, 7-6 (4), 6-3.
"But the guy's won a lot of Grand Slams, and he knows how to win them", Millman said after losing to Djokovic in the quarter-finals.
But Djokovic is almost six years younger than Federer and - prior to a two-year barren spell that was ended by his Wimbledon triumph - had been winning slams at an average of almost two a season.
Argentine third seed Del Potro had been seeking first Grand Slam title since 2009.
A focused Djokovic continued to apply pressure in the third set until Del Potro finally cracked on serve to fall behind 5-3, with the Serb closing out the match with a ferocious overhead victor. But when Djokovic faced three break points in the following game, an epic that required 22 points to settle, he saved them all.
An eight-time finalist in NY who had only previously lifted the trophy in 2011 and 2015, the Serbian was made to work hard on Sunday as Del Potro came from a break down in the second and third sets. After the third round, he did not lose a single set as he was able to outlast a fan favorite in Del Potro.
He said: "I think I did a good tournament, but this is a tough loss for me".More news: Map of North Carolina Showing Latest GFS, European Models
The Serb admitted it was special, adding: 'Pete Sampras is one of the biggest legends ever to play the game.
"I was sad for Serena because she's a great champion", del Potro said.
"I thought of tennis, thought of the emotion that tennis provokes in me in a way".
Former French Open and Australian Open champion Li Na has been credited with sparking a tennis revolution in China, and Wang said: "I watch some matches about Li Na".
But everything - his health, his backhand and his confidence - has come together this season for the Argentine to have his best year since 2009, when he won the US Open and snapped Roger Federer's 40-match NY win streak in the final.
Backed by his singing boys, Del Potro roared back in the set by breaking Djokovic to go back on serve.
Thereafter, there were two service holds and the set was poised at 3-2 and suddenly, Flushing Meadows saw a massive momentum shift. He has come to the conclusion that his time away during rehabilitation re-energized an already rousing career. I had my opportunities during second and third set. Djokovic, up a set, was serving at 3-4 when the two proceeded to play 23 points. He has reached the semi-finals or better at each of the last ten US Open Championships in which he has taken part.
The 2.4 overnight rating is up 32% from previous year (1.9 for Sloane Stephens and Madison Keys of the U.S.) and 79% higher than 2016 (1.4 for Angelique Kerber vs. Karolina Pliskova).