More than a day has passed but the tennis world still hasn't recovered from Roger Federer's shock US Open loss against world No. 55 John Millman.

Known for his impossibly strong forehands and ability to manoeuvre himself with great agility, Federer struggled badly in the match and looked slow, sluggish and frustrated throughout.

Eventually, he went down 3-6, 7-5, 7-6 (7), 7-6 (3) and succumbed to a fourth-round loss - his earliest exit at the US Open since 2013.

At the post-match media conference, Federer was honest in the assessment of his performance. "For some reason, I just struggled in the conditions tonight," Federer was quoted as saying at the conference after the match.

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

"It's one of the first times it's happened to me. I just thought it was very hot tonight. I felt I couldn't get air. There was no circulation at all. It's uncomfortable, sweating more and more and more and more as the match goes on. You lose energy as it goes by. When you feel like that, everything is off."

While the humid and sapping conditions might have played a part in affecting Federer's performance, was there more to it as well? Was it just an off day at the office for one of the world's greatest Tennis players? Or perhaps it was a sign of things to come?

While Federer began 2018 in style, his form has been a tad off lately. He is 37 and has been relentless over the years. It is almost impossible to sustain the momentum that he has developed over the years for so long.

These days, however, it takes Federer longer to come back to form after hitting a rough patch. Lest one forget he hadn't won a Slam between 2012 and 2017 and that must have taken a toll on him.

The challenge for Federer now would be to shake this shock defeat and continue to focus ahead. Federer won't be playing in any clay events this year so he can manage his workload well. The US Open loss would be fresh in his mind and so would be the loss to Juan Martin del Potro in the final at Indian Wells. He has also not reached a Grand Slam semifinal since February or won a Masters Series event.

Clearly, the 37-year-old five-time champion's performance has been sporadic of late. For any other player, this might still have been considered a good season. However, Federer is a tennis behemoth and one expects better from him. The coming season will tell us if his shock loss in the US Open was just an aberration or is there more to it.