"It was just tough". I travel 10 and a half months a year and that's tough.
The pair are actually friends and regularly train together, but Millman said that his victory was all business. The internet was still abuzz, before the Swiss could even walk out on court, about that around-the-netpost shot he - nearly casually - played against the talented Nick Kyrgios at the same venue a few days earlier.
It's not goodbye from him just yet. He was like, Yeah, absolutely, I'm ready to come.
Maybe this time, just maybe, there will be a few more in the stands looking to buck up the Australian.
Millman, who trained with Federer in Switzerland before the grass-court season, was in shock at having beaten one of his idols.
The contest was over shortly before 1 a.m. when he hit a tired forehand wide.
Following an injury interrupted campaign in 2017, Millman closed out the calendar year ranked 128th overall.
"Midway through the second set, I realised there was something wrong, and noticed he was sweating way more than usual", she said in commentary for ESPN.
Djokovic, who ended a 54-week title drought with his 13th Grand Slam title at Wimbledon, said he was relieved to get past 68th-ranked Sousa in straight sets, the searing afternoon heat seeming more of a problem than his Portuguese opponent.
Djokovic and Federer seemed set for a last-eight blockbuster, but Millman came roaring back in his clash with the Swiss veteran - a five-time champion - to astonishingly triumph 3-6 7-5 7-6 (9-7) 7-6 (7-3).
But the Aussie regathered himself in the second set and had the Swiss master's measure in the third and fourth, taking the final two sets in tie-breaks.
"I'm probably in a little bit of disbelief", said Millman.
"I'm very aware he didn't have a great day in the office". He cracked, double faulting again and making two more unforced errors, as Millman roared back to level the match at 1-1.
"It was a slightly intimidating environment", he said. "He started playing well in the last couple of games, but happy to finish in three sets today".
"One, he's such a good tennis player but I'm more proud of how he acts, the person he is and how he treats other people".
Federer wasted two set points in the 10th game, giving Millman a break chance when he missed an easy volley then double-faulting on break point.
Millman is just the second player to defeat Federer in a Grand Slam, and the first to do it since Patrick Rafter back in 1999. He will next face Serbia's Novak Djokovic.
"He's an incredible player".
Millman paused, then added, "I don't know whether to go Gurley or Le'Veon Bell".