WHAT HAPPENED: Tommy Robredo, meet John Millman. The 29-year-old Aussie halted Roger Federer’s hopes for a sixth US Open title on Monday night, upsetting the five-time champion 3-6, 7-5, 7-6, 7-6 in Arthur Ashe Stadium. The 37-year-old Swiss had made 12 of the past 13 US Open quarterfinals, with the only miss coming in 2013 when he fell to Spain’s Robredo.

But the second seed struggled in the humid conditions and never found a way to disrupt the resilient Millman, who turned the fourth-round match into a physical baseline affair. Federer had set points in the second and third sets but couldn't convert. He shortened points, serving and volleying and mixing in drop shots. But Millman, who was playing in his first Grand Slam fourth-rounder, swung freely for the biggest win of his career.

“It was very hot tonight. Was just one of those nights where I guess I felt I couldn't get air. There was no circulation at all. I don't know, for some reason I just struggled in the conditions tonight. It's one of the first times it's happened to me,” Federer said.

September 3, 2018 - Roger Federer exits Arthur Ashe Stadium after losing against John Millman during the 2018 US Open. (USTA/Brad Penner)
Photo by:  (USTA/Brad Penner)

READ MORE: Federer, Millman react to their 2018 US Open Round-of-16 clash 

“But John was able to deal with it better. 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, as well, you start missing chances, and I had those. That was disappointing. But, look, at some point also I was just happy that the match was over, I guess.”

Federer had been 40-0 against players ranked outside the Top 50 of the ATP Rankings at the US Open. But Millman, No. 55, ended the streak and snapped his own 10-match losing streak against Top 10 players (0-10 overall).

 “I’m probably in a little bit of disbelief. I have so much respect for Roger and everything he’s done for the game. He’s been a hero of mine. And today he was definitely not at his best, but I’ll take it,” Millman said.

Federer broke to love in the second game and rolled through the opening set in only 33 minutes. He had to save seven break points to start his serving in the second, but he prevailed in the 14-minute game and his team stood to applaud the moment.

Millman, however, didn’t go away, even after he was broken in the fifth game of the second. Federer had two set points while serving at 5-4, 40-15, but Millman forced it to deuce, and Federer double-faulted to hand back the break. The Swiss landed only 31 percent of his first serves in the second set (17/55), and as he served to stay in it at 5-6, a backhand winner from Millman brought up set point, and Federer lifted a backhand long to even the match.

September 3, 2018 - John Millman and Roger Federer meet at net during the 2018 US Open. (USTA/Jennifer Pottheiser)
Photo by:  (USTA/Jennifer Pottheiser)

“I felt like a bit of a deer in headlights to begin with, to be honest with you. The feet weren’t moving. Roger had it on a string; he was manipulating me around the court,” Millman said. “But I got out of a tough second set and really found my feet and started to be a little bit more aggressive. I think I started to serve really well and probably capitalized a little bit on Roger having an off service day.”

Federer had been 40-0 at the US Open against players ranked outside the Top 50.

The 20-time Grand Slam champion Federer hadn’t dropped a set all tournament, beating Japan’s Yoshihito Nishioka, Benoit Paire of France and 30th seed Nick Kyrgios in straights. But Millman presented a different challenge, darting from side to side and forcing Federer to win the points. Millman finished with only 28 unforced errors, compared to Federer’s count of 77.

Federer tried to take advantage of Millman’s deep return position. But too often he had to rely on his second serve to pressure Millman, and the Aussie’s return improved throughout the match. Millman also chased down any bouncy drop shots for winners.

Federer had a set point at 6-5 in the third-set tiebreaker but Millman erased it, and the Aussie clobbered a forehand approach that Federer couldn’t handle for a two sets to one lead.

Last year, Federer had won two five-setters in Ashe – coming back from two sets to one down against Russian Mikhail Youzhny and beating American Frances Tiafoe in five sets. He broke Millman to roars from thousands of fretting Fed fans and served for a 5-2 lead in the fourth.

play video AI Match Highlight: Millman vs. Federer - Round 4

But Millman broke back immediately. In the tiebreaker, at 1-2, Federer twice double-faulted, his ninth and 10th of the match, and a missed forehand brought up five match points for Millman. He converted his third when Federer lifted another forehand long.

WHAT IT MEANS: The potential Djokovic-Federer quarterfinal that fans been anticipating ever since the draw was released? Not happening. Millman crashed what would have been Djokovic and Federer's 47th meeting, and seventh at the US Open. The Aussie, however, will not be apologizing anytime soon.

Millman won three games when he played Djokovic on the grass courts at The Queen's Club in London earlier this year. But Millman had been 0-1 against Federer before Monday night as well.

"The one thing I can control is the fight within me. I’ve always done that throughout my career,” Millman said.

MATCH POINT: Federer had been 40-0 at the US Open against players ranked outside the Top 50. Millman is ranked No. 55 in the ATP Rankings.