"I don't have a crystal ball, do you?"

Roger Federer was terse when asked if he felt he would have more opportunities to win grand slams like the one he just let slip at the US Open, with Novak Djokovic out of the draw and the Swiss having a clear path to a potential final with Rafael Nadal.

He is right, of course. He does not have a crystal ball and neither does anyone else. However, he certainly could have used one ahead of Tuesday's match with Grigor Dimitrov, as even the most confident of fortune tellers could not have envisaged what the Bulgarian produced at Arthur Ashe Stadium.

Dimitrov stands as one of a growing number of once highly regarded ATP Tour players who have been unable to live up to their potential. So lofty was the opinion of Dimitrov's considerable talents, that he was once nicknamed 'Baby Fed'.

There have been considerable highs in Dimitrov's frenetic career that justified his reputation. His 2014 Wimbledon quarter-final win over then-defending champion Andy Murray was a supremely accomplished performance against an opponent playing with the vociferous backing of the home crowd.

His 2017 run to the Australian Open semi-finals was another strong hint at a breakthrough but it again appeared to be a false dawn, and there was nothing going into the last eight at Flushing Meadows to suggest he would be able to topple Federer.

Federer had won their previous seven meetings and had looked imperious in swatting Daniel Evans and David Goffin in the third and fourth rounds, while Dimitrov was playing in his first Tour-level quarter-final since January.

Even with Dimitrov having played extremely well to win the second set, there were few inside Ashe who expected it to be little more than a blip in the five-time champion's progression to the last four, with that assessment seemingly set to be vindicated when he took the third.

Yet, as Federer later said, this was "Grigor's moment", and he made sure of that in a tremendous fourth-set performance encompassing everything that had once led to him being considered the heir to Federer's throne.

There was power, variety, excellent movement and there were passing shots, oh so many passing shots, continually thundered beyond an ailing Federer off the forehand and backhand sides.

Federer insisted he was not surprised by Dimitrov's performance.

"It's the Grigor I expected. He has returned against me in the past also a little bit further back. He has been in, chipped, come over. He has the arsenal to do all sorts of things. He used it all tonight to great effect," said the 38-year-old.

That opinion is probably only shared by Dimitrov, as the sense of shock inside the world's biggest tennis stadium was palpable as the 28-year-old wore Federer down to the extent that he had to take a medical time-out for a back injury at the end of the fourth set.

Dimitrov displayed incredible character and endurance in doing so. Service games on both sides played out as mini-dramas within a fascinating thrill ride, with the Bulgarian's desire to make Federer play as many balls as possible paying dividends in the seventh game of the fourth. 

Federer held after a game that featured eight deuces and in which he had to save seven break points. Dimitrov may have been unable to get the double break, but he knew the damage had been done.

He said: "I think even when I lost that game, I was actually smiling going through the changeover because I was [thinking], 'That game must have hurt him a lot.' For me, it actually filled me up.

"After that fourth set, I felt also he kind of needed a little bit of a break, as well. I kept on pushing through. I think in the first game in the fifth, I put so many returns back, pretty much all the returns, so he had to go. He wanted to keep the points really short. I used every single opportunity I had."

That was the difference between the Dimitrov of old and the one that stunned a hugely pro-Federer crowd, perceptiveness and patience. He knew his opponent was struggling, he knew he did not have to swing for the fences. He did not have to go for the kill, because he knew the kill would come to him.

It came in rapid fashion in the fifth set as Dimitrov secured a 3-6 6-4 3-6 6-4 6-2 triumph that will go down as one of the biggest upsets in recent memory. It was a result virtually nobody expected from the world number 78 but, regardless of what he does from here in New York, Dimitrov's shock defeat of Federer will make sure nobody doubts his ability to deliver on the grand slam stage again.