The unseeded and 55th-ranked Millman pulled the Labor Day stunner, rallying from a set down to oust Federer 3-6, 7-5, 7-6 (7), 7-6 (3) at Arthur Ashe Stadium in a 3-hour and 34-minute marathon that stretched into early Tuesday morning.
He becomes the second ever Aussie to defeat Federer in a Grand Slam, following Pat Rafter a whopping 19 years ago.
He said: "I was playing one match at a time because I didn't have any confidence and I was still anxious about my wrist".More news: Tottenham's Son Heung-Min Avoids Miltary Duty After Asian Games Triumph
Before their match, Roger Federer said he had a great deal of respect for his Australian counterpart. Today he wasn't at his best but I will take it.
Millman cursed himself for missing one set point but threw everything into a series of forehands to bring up a second and forced Federer into another error.
Federer, indeed, did not play as well as he had in the opening three matches, when he didn't drop a set.
Following an injury interrupted campaign in 2017, Millman closed out the calendar year ranked 128th overall.More news: Alberta government to provide financial update on heels of Trans Mountain bombshell
After a routine-looking first set the Australian took advantage of Federer's awful first serve percentage to dictate the play and come back into the match.
Many labelled Federer's performance in the match as one of the worst of his career. He wasn't in awe of the 20-time Grand Slam champion after a shaky start. "Definitely a player I've looked up to throughout my career", he said before the match. 'So the draw, it is what it is, and I've got enough stuff to worry about anyway in the earlier rounds. "But I have never been a fan of playing someone's reputation".
Things grew tougher in the second as a sluggish Djokovic laboured before again clinching the set with a break.
But the two set points came and went and a double fault gave Millman the break back.More news: California Blockchain Bills Would Enable New Understanding, Private-Sector Trial
The next foe? That could be Roger Federer. When Djokovic stole a break midway through the set to get ahead 3-2, he ran away with the lead and finished with a final break to seal the match.