Health? Desire? Nah, but the heat was a major problem for Roger Federer

Flashback: Federer falls to Millman in four sets (2:34)

No. 2 Roger Federer wins the first set but loses the next three, including two via tiebreak, in falling to John Millman in the US Open's fourth round. (2:34)

NEW YORK -- It was only two days ago that we lauded Roger Federer as a Jedi master for his flawless win over Nick Kyrgios. We marveled at Federer's impossible forehands and how he float-dances from baseline to net. On Monday night, however, against world No. 55 John Millman, Federer racked up 76 unforced errors.

Federer struggled on serve and looked slow, sluggish and frustrated throughout a 3-6, 7-5, 7-6 (7), 7-6 (3) fourth-round loss, his earliest exit at the US Open since 2013. It was a stunning 180 by the 20-time Grand Slam champ, who had all in attendance wondering what had gone so wrong.

"For some reason, I just struggled in the conditions tonight," Federer said in his postmatch media conference, which started at 2 a.m. ET. "It's one of the first times it's happened to me. I just thought it was very hot tonight. I felt I couldn't get air. There was no circulation at all. It's uncomfortable, sweating more and more and more and more as the match goes on. You lose energy as it goes by. When you feel like that, everything is off."

It was an honest assessment from a player not known for showing his hand or conceding weakness, especially after a losing effort.

"John was able to deal with it better," Federer said of the conditions. "He maybe comes from one of the most humid places on earth, Brisbane. I knew I was in for a tough one. Maybe when you feel like that, you start missing chances, and I had those. That was disappointing. Look, at some point I was just happy that the match was over."

That's a stunning, and perhaps worrisome, confession from one of the most competitive athletes on the planet, who has said he will continue to play the game as long as the fight is in him to grind it out on tour.

Monday's Labor Day match started like a typical day at work for Federer. He won the first three games, smashed three aces and needed only 33 minutes to take the first set 6-3. It looked like he would make short work of Millman, who called Federer "my hero" after the Aussie's third-round win over Mikhail Kukushkin on Saturday and said he was just happy to be playing at Arthur Ashe Stadium for the first time.

"I felt like a deer in the headlights to begin with," Millman said of his play against Federer in the first set. "My feet weren't moving, and Roger had it on a string and was manipulating me around the court."

Federer served for the second set up 5-4 and 40-15, but Millman rallied to even the score at 5-all.

"Go to work, John!" a fan yelled as the stunned crowd -- which included Vogue editor-in-chief Anna Wintour, former President Bill Clinton, Golden State Warriors head coach Steve Kerr and comedian John Mulaney -- fell silent. "There's more work to be done! Get. To. Work. John!" Millman would win the next two games to even the match at one set apiece. When he held serve to go up 3-2 in the third set, fans started a round-the-stadium chant of, "Aussie, Aussie, Aussie ... Oi, oi, oi!"

Then Millman won the third-set tiebreaker and the crowd remembered why they were there. They remembered what was at stake. A Millman win would mean no Federer-Djokovic quarterfinal on Wednesday night, no rematch of last month's final in Cincinnati. A Federer loss in the fourth round of the US Open would begin whispers of that "R" word again, despite Federer's insistence he has no plans to leave the game anytime soon. A Federer loss like this might be unwanted proof the Force is no longer strong in the Jedi's 37-year-old legs.

The crowd was willing to cheer David to take a game or two against Goliath -- but not two sets. For the remainder of the match, the crowd was rowdy in its support of the Swiss giant. "Let's go, Roger!" "Federer forever!" "Roger, remember: You're No. 1!" (He's currently ranked No. 2.)

Then in the fourth-set tiebreaker, as Millman served for the match, the crowd fell quiet. There was little more to do but watch as the Aussie underdog completed the inevitable and felled his hero. It was the first time Millman beat a top-10 player in his career.

"I found my feet and started to be aggressive and serve really well, and I capitalized on Roger having an off service day," Millman said. "I felt a little bit guilty because he didn't have his best day. I know that. I'm very aware he didn't have a great day in the office. Probably to beat him, I needed him to have an off day and I needed to have a decent, good day."

The burning question is whether this was simply an off day at the office for one of the game's greatest or a sign of what's to come. Admittedly, that's hardly an original query. Let's not forget Federer failed to win a Slam between 2012 and 2017 and then won two majors at age 36.

The start to 2018 felt like an extension of Federer's brilliant 2017 season. He defended his title in Melbourne, his 20th Grand Slam title. But he hasn't seen a Slam semifinal since.

That's not to say this season has been a bust. He won in Stuttgart, lost to Juan Martin del Potro in the final at Indian Wells, lost to Kevin Anderson in a five-set marathon in the quarters at Wimbledon and lost to Djokovic in the final at Cincinnati. By anyone else's standards, this has been a successful season. But Federer is not anyone else. When he slips, we question why. Is it his age? His health? His fitness? His desire?

"It was just hot," he said. "No shame there."