So much for Roger Federer against Novak Djokovic in the 2018 U.S. Open.

John Millman played spoiler to what would have been one of the most highly anticipated meetings of this year's tournament with a 3-6, 7-5, 7-6 (7), 7-6 (3) victory over the second-seeded Federer in Monday's fourth-round showdown at Arthur Ashe Stadium. Millman will now play the sixth-seeded Djokovic in the quarterfinals.

According to the ESPN broadcast, this was Millman's first-ever win against a top-10 player, and he did it on one of the sport's biggest stages against an all-time legend.

It was also a shocking loss by Federer's historically great standards:

The result was largely possible because of an uncharacteristically poor performance from the Swiss star. Federer had a jarring 76 unforced errors to just 28 from Millman, per the tournament's official website, which was too much to overcome even though they each won the same number of break points (three).

"I have so much respect for Roger and everything he's done for the game and he's been a hero of mine and today he was definitely not at his best but, you know, I'll take it," Millman said after the match, per Christopher Clarey of the New York Times.

Federer figured to cruise if he was at his best as a 20-time Grand Slam champion, but he also needed the entire three sets when he won his only other match against Millman in 2015. The two are more familiar with each other than the lack of history suggests, as Federer revealed he has trained with Monday's opponent in the past.

"I'm a big fan of John. Maybe not that many people know him but he's a real hard-working guy out there and those are the guys who have all the respect from us top players in the locker room," he said, per Australian Associated Press (h/t ESPN.com). "He's just come to Switzerland to train with me before the grass-court season so we know each other quite well, a super friendly guy, and I'm very happy for him that he's gotten so far."

He's surely not happy Millman made it past him, especially since Monday was a drastic change from the first three rounds when he didn't drop a single set and resembled the version of himself that won this tournament five straight times from 2004 through 2008.

Federer wasted little time setting the tone and broke Millman's first service game on his way to a straightforward first-set win before things went off course.

Unforced errors and missed opportunities allowed Millman to break twice in the second set after falling a break behind, and Federer couldn't even force a tiebreak despite the fact he served for the set up 5-4.

Federer was able to force a tiebreak in the third set by remaining on serve and again in the fourth set after they exchanged breaks, but it did him no good, as Millman came through with the match hanging in the balance both times.

Attention now turns to the clash between Djokovic and Millman in the quarterfinals, as the Australian looks to stun the tennis world again with a victory over a marquee name.

Djokovic won the only previous match between the two earlier this year by cruising in straight sets, but the Millman who took the court against Federer didn't look like someone who will go out without a challenge.

If Djokovic struggles at all with unforced errors and isn't able to take advantage of opportunities to put the match away, Millman proved he is capable of beating one of the best the game has ever seen.