Djokovic was becoming frustrated after suffering a fall and having dropped serve to trail 5-6, he hit a ball reasonably hard to the back of the court, striking a female line judge above the shoulders.
He immediately apologised and stood over her as she appeared in distress on the floor.
Tournament referee Soeren Friemel then came out on to the Arthur Ashe Stadium and spoke to chair umpire Aurelie Tourte before a lengthy discussion with the 33-year-old Serb, a three-time former U.S. Open winner.
Djokovic was clearly making the point that he had not intended to hit the official but under the rules of tennis his fate was already sealed.
The Grand Slam rules state: “Players shall not at any time physically abuse any official, opponent, spectator or other person within the precincts of the tournament site.
“The referee, in consultation with the Grand Slam chief of supervisors may declare a default for either a single violation of this code.”
Despite his pleadings, Djokovic was eventually disqualified before walking over to shake hands with a stunned Carreno Busta and trudging off to face the music.
It was an incredible end to the top seed’s hopes of winning an 18th Grand Slam title — something that had looked increasingly likely as the fortnight progressed.
“It’s the right decision,” Tim Henman, who was disqualified from Wimbledon for a similar incident in 1995, told Amazon Prime. “He is not aiming for the line judge, but has hit the ball away and you have to be responsible for your actions.”
Former champion Mats Wilander, analysing the action for Eurosport, said it was very unlucky for Djokovic but that it had been the correct decision.
“You are not allowed to do that,” he said. “It’s as much bad luck as you can have on a tennis court. He didn’t just roll the ball back to the ball kid, that’s the bottom line.
“He hit it harder than he intended to, obviously a complete accident. It was a sign of frustration, yes. A little bit. But it doesn’t matter, you are not allowed to do it.”
The incredible development has blown the draw wide open, especially with defending champion Rafa Nadal and Roger Federer absent this year. Nadal opted not to travel to New York because of concerns over the COVID-19 pandemic, while Federer is recovering from knee surgery.
A world number one being defaulted sent shockwaves around Flushing Meadows which has witnessed many crazy events down the years.
“I’m a little bit in shock right now, to be honest,” Alexander Zverev told reporters after his own fourth-round win over Alejandro Davidovich Fokina.
“If he would have hit it anywhere else, if it would have landed anywhere else, we are talking about a few inches, he would have been fine.”
Carreno Busta will now face a quarter-final against either Denis Shapovalov or seventh seed David Goffin.
A tournament spokesman told Reuters the line judge appears to be okay and was not brought off-site. Her identity is not known.