Australian John Millman has stunned the tennis world with a fourth-round upset over the legendary Roger Federer.

The unseeded Millman won 3-6 7-5 7-6 (9-7) 7-6 (7-3) to secure a quarter-final against 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 Swiss master at a major tournament.

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 to Federer in 20 straight grand slam matches over almost 20 years.

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.

Federer won the opening set in 33 minutes and served for the second at 5-4, before Millman saved two set points to level at 5-5.

“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.”

The World No.55 ultimately won the second set 7-5 and claimed the third in a tie-breaker but dropped serve to trail 4-2 in the fourth.

A shattered Roger Federer. Photo: AAP

But Millman immediately broke back, as the fourth set went back on serve and proceeded to another tie-breaker, which he won 7-3.

Earlier Djokovic had sweated his way to a 6-3, 6-4, 6-3 fourth round win over Joao Sousa as the heat returned to the US Open.

A Federer-Djokovic clash had been much anticipated given the pair have seven titles at Flushing Meadows between them.

-with AAP