Five-time champion loses to Australia’s 55th-ranked John Millman in the fourth round of the year’s final Grand Slam.

Swiss tennis star Roger Federer has been knocked out of the fourth round of the US Open following an upset defeat at the hands of unseeded Australian John Millman.

The world number two looked below par and at times sluggish on a humid Monday evening at Flushing Meadows, New York, losing 3-6, 7-5, 7-6 (7), 7-6 (3) to his 55th-ranked opponent after 3 hours and 34 minutes.

This is the first time Federer, 37, has lost to a man ranked outside of the top 50 at the year’s final Grand Slam tournament, where has won five titles.

It is also only the second time in Federer’s past 14 appearances at the US Open that he has lost before the quarter-finals.

With the defeat, the 20-time Grand Slam champion failed to set up a highly-anticipated quarterfinal match-up with his Serbian rival Novak Djokovic, seeded sixth.

Absolutely stunning upset as @johnhmillman defeats Federer 3-6, 7-5, 7-6, 7-6 under the lights in Arthur Ashe Stadium! Millman's first top ten victory lands him a spot in the QF against Djokovic…#USOpen pic.twitter.com/4DPEOJpJw7 — US Open Tennis (@usopen) September 4, 2018

Unseeded Millman, 29, who had never made it past the third round at a major until last week, said he froze in the first set, but was able to take control by relying on his fighting qualities.

“I’m probably in a little bit of disbelief,” he said in a courtside interview. “I have so much respect for Roger and everything he’s done for the game.

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“He’s a hero of mine. Today, he was not at his best but I’ll take it.”

Federer, who won the Australian Open, the year’s first Grand Slam in January, committed 76 unforced errors and hit 10 double faults, including twice in a row during the final tie-break.

The Swiss player was looking to capture his sixth US Open title and extending his record of 20 Grand Slams.

Millman will now face current Wimbledon champion Djokovic, who beat Portugal’s Joao Sousa in the fourth round, on Wednesday.

Grand Slam championships are the four most important tennis events held annually in France, the United States, Britain and Australia.