In what is one of the biggest upsets of the US Open, world number two Roger Federer was stunned in four sets by unseeded John Millman on Tuesday, September 4 in New York.

Federer, as pointed out by Millman and fans, was far from his best as he made a whopping 77 unforced errors, including 10 double faults.

The energy-sapping heat in New York also played its part as the 20-time Grand Slam champion was tiring towards the end of his 3-6, 7-5, 7-5, 7-6(3) defeat at the Arthur Ashe Stadium.

Surprisingly, Federer struggled with his serves, managing to get in only 49% of his first serves, compared to Millman's 64. Notably, he had set points in the second and the third set but failed to convert them on both occasions.

"I am probably in a little bit of disbelief. I have so much respect for Roger, everything he has done for the game. He's been a hero of mine. But today he was definitely not at his best. But I'll take it," an elated Millman said. 

Roger Federer
Roger Federer suffered a shock defeat in the fourth round of US Open 2018.Julian Finney/Getty Images

This was also Federer's first defeat to a player ranked outside the top 50 of ATP rankings at the US Open, which he hasn't won since 2008. On the other hand, Tuesday's win was Millman's first against a top 10 player.

Unlike last year where Federer reigned supreme with seven titles, including two Grand Slams, the 37-year-old has struggled for consistency in 2018.

After defending his Australian Open title in January, Federer failed to make an impact in the Sunshine tournaments, which he had won last year.

He had skipped the clay court swing of the season stumbled in the quarter-final of Wimbledon, after which he lost in the final of Cincinnati Masters last week.

Federer's under-par show in New York has brought disappointment to fans, who were expecting the five-time champion to add another US Open crown.

Nonetheless, quite a few of them are optimistic that the Swiss great, who is in the twilight of his career, will be back at the Flushing Meadows next year.

Roger Federer
Roger Federer made 77 unforced errors during his fourth-round defeat at US Open 2018.Julian Finney/Getty Images

IBTimes, India has compiled a list of reaction to Federer's shock defeat. Check them out here.