IN one of the greatest upsets in US Open history John Millman, the Brisbane journeyman, has turned the tournament upside down with a colossal fourth-round upset win over 20-times grand slam champion Roger Federer.

In a life-changing performance, Millman left Federer - and fans - at Arthur Ashe Stadium shellshocked with a 3-6 7-5 7-6 (9-7) 7-6 (7-3) comeback victory to book a quarter-final date with another of tennis’s all-time greats, Novak Djokovic.

Not since Pat Rafter handed a tantrum-throwing Federer a tennis lesson on the then-18-year-old’s grand slam debut at Roland Garros in 1999 had an Australian conquered the great Swiss at a major.

Since then, nine Australians - including Nick Kyrgios only two days ago, Mark Philippoussis in the 2003 Wimbledon final and Lleyton Hewitt seven times - had fallen prey to Federer in 20 straight grand slam matches over almost 20 years.

But on the very same court on which Federer destroyed Hewitt 6-0 7-6 6-0 in the 2004 US Open final, Millman stared down the greatest player of the professional era to send the world No.2 crashing to his earliest grand slam defeat in almost four years.

“I’m probably in a little bit of disbelief,” Millman said after smashing Federer’s 40-from-40 record against non-top 50-ranked rivals at the US Open.

“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.”

The upset for the ages looked remote when Federer broke Millman - playing his his first fourth-round match at a slam at age 29 - in the second game of the match en route to taking the opening set in routine fashion.

“I felt like a bit of a deer in headlights to begin with, to be honest with you,” Millman said.

“The feet weren’t moving. Roger had it on a string, he was manipulating me around the court.

“I got out of a tough second set and really found my feet and started to be a bit more aggressive.

“I started to serve really well and probably capitalised a little bit on Roger having an off service day.”

While few gave the Brisbane baseliner a chance, Federer had full respect for Millman, having invited him to Switzerland before Wimbledon this year to prepare for the grass-court season.

But he could not have expected Millman to maintain such a high level for more than three and a half hours.

In addition to earning a crack at Djokovic on Wednesday for an improbable place in the semi-finals, Millman is guaranteed a $US475,000 ($660,000) pay day and a rankings rise to No.37 in the world.

Should he beat Djokovic, he will leapfrog Kyrgios to become Australia’s new No.1.

And all this after Millman feared his career was over following two shoulder surgeries and then a groin operation last year.

The winner of the Djokovic-Millman quarter-final will play seventh-seeded 2014 champion Marin Cilic or Japanese star Kei Nishikori for a spot in Sunday’s title match.