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 38 minutes.

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,

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, 29, 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 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.

Warning signs were clear for Federer

After not dropping a set in his opening three matches, Federer looked on his way to another controlled victory when he eased the opener in 33 minutes.

However, the warning signs were clear from the start of the second set.

Federer’s reliable serve has been the bedrock of his game over the years but an uncharacteristically erratic display allowed Millman to fight back and earn a memorable win.

The world number two produced a contender for one of the ugliest service games of his career, unable to find his timing as he landed just six of his 24 first serves in a 14-minute second game.

He somehow managed to hold after fighting off seven break points, before breaking Millman’s serve himself to lead 3-2.

But, with his first serve percentage down at 31%, Federer still looked uncomfortable.

He blew two set points when serving at 5-4 and a double fault at break point gifted Millman the game and the Australian broke again as he won four games in a row to level the match.

Willed on by a supportive crowd on Arthur Ashe Stadium, Federer managed to steady his service game in a tight third set and missed a set point in the tie-breaker when Millman hit an outrageous 100mph second serve down the middle.

The Australian wasted one of his own with a long return as Federer battled to stay in the set, only to take his second chance and leave Federer facing an uphill task.

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