Swiss dumped out in four sets by Australian
Millman had been given barely a sniff before the match, with excitement already building for a last-eight clash between Federer and Djokovic.
The 29-year-old, who had never previously beaten a top-10 opponent, appeared to have even less chance of winning with Federer a set and a break up and holding two set points, but he somehow clawed his way back to clinch a 3-6 7-5 7-6 (8/6) 7-6 (7/3) victory.
While Millman produced the performance of his life, Federer gave one of his poorest performances on the big stage, struggling with all parts of his game and making 76 unforced errors.
It was the 37-year-old’s earliest loss at a slam since the Australian Open in 2015 and the first time he has ever lost to a player ranked outside the top 50 in New York.
Federer had struggled with his timing in the early stages of his previous match against Nick Kyrgios before clicking into gear, but against Millman he just could not find his game.
His serve, in particular, was way off during the first two sets. In the second game of the second set he faced seven break points and landed only four of 22 first serves but still held.
And at a set and 5-4 up with two set points on his own serve, it seemed this would simply be a case of Federer grinding out a relatively straightforward victory.
But the two set points came and went and a double fault gave Millman the break back. Two games later, Federer was facing set point against him, and he blazed a backhand long.
Federer, renowned as the man who never sweats, was perspiring heavily and it was not entirely down to the extreme humidity.
Millman, ranked 55, could not be less like his compatriot Kyrgios. A universally popular player who has built a successful career, despite several serious injury setbacks, on never giving an inch.
And, as Federer continued to struggle, the Australian’s confidence grew. Had the 20-time grand slam champion’s serve not improved, he might have lost the third set before the tie-break.
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.
The crowd on Arthur Ashe, who were overwhelmingly behind the Swiss, could not believe what they were seeing.
Federer had so little confidence in his groundstrokes he was playing drop shots nearly every point, with mixed success. A chink of light arrived with a break for 4-2 only for more mistakes to hand it straight back.
Successive double faults put him firmly on the back foot in the tie-break and there was no way back.