US Open 2018: Roger Federer knocked out by Australian John Millman

Roger Federer has been knocked out of the US Open in the fourth round by world number 55 John Millman.

The 37-year-old Swiss had prioritised winning a sixth title at Flushing Meadows but was beaten 3-6 7-5 7-6 (9-7) 7-6 (7-3) in three hours 35 minutes, BBC Sport reports.

It is the first time Federer has lost to a player ranked outside of the top 50 at the US Open.

Australian Millman will play two-time champion Novak Djokovic in his first Grand Slam quarter-final on Wednesday.

Millman, 29, is the only unseeded player left in the last eight of the men's draw.

"I'm probably in a little bit of disbelief," said Millman.

"I have so much respect for Roger and everything he's done for the game. He's been a hero of mine."

Second seed Federer, bidding for a record-extending 21st Grand Slam, missed set points in the second and third sets before capitulating in the fourth-set tie-break.

Millman had never beaten a top-10 player before but suddenly found himself with five match points against arguably the greatest male player of all-time.

After Federer rallied to save two of the them, Millman completed victory at almost 01:00 local time in a humid New York when the out-of-sorts Swiss lumped a forehand long.

It was Federer's first defeat in 41 matches at the US Open against a player ranked outside the top 50.