NEW YORK - World number one Novak Djokovic was sensationally disqualified in the US Open fourth round on Sunday after striking a line judge with a ball following a point during the first set of his match against Spain's Pablo Carreno Busta.

Djokovic was becoming frustrated after squandering three set points when Carreno Busta served at 4-5, 0-40, then suffering a fall. Having dropped serve to trail 5-6, he swatted a ball reasonably hard to the back of the court, inadvertently striking a female line judge in her throat.

With the line judge screaming out in pain as she fell to the ground, a horrified Djokovic ran over to her and apologised.

Tournament referee Soeren Friemel came out on to the Arthur Ashe Stadium and spoke to chair umpire Aurelie Tourte and Andreas Egli, the Grand Slam supervisor, before a long chat with the 33-year-old Serb, a three-time former US Open winner.

Djokovic was clearly making the point that he had not intended to hit the official and was overheard to say "she doesn't have to go to hospital for this".

After 12 minutes of pleading, Djokovic's fate was 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."

Djokovic eventually walked over to shake hands with a stunned Carreno Busta and trudged off to face the music, although he later left the grounds in a black Tesla without attending a press conference.

He later posted an apology on Instagram: "This whole situation has left me really sad and empty... I‘m extremely sorry to have caused her such stress. So unintended. So wrong.

"As for the disqualification, I need to go back within and work on my disappointment and turn this all into a lesson for my growth and evolution as a player and human being. I apologise to the @usopen tournament and everyone associated for my behaviour."

It was a shocking end to the top seed's hopes of winning an 18th Grand Slam title and moving within two of the men's record total won by Roger Federer -- something that had looked increasingly likely as the fortnight progressed.

It also brought to an end Djokovic's 26-0 winning run since the start of the year, albeit in bizarre circumstances.

The USTA issued a statement saying that Djokovic would lose all the ranking points earned from the event and will be fined the $250,000 -- his prize money for reaching round four.

"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."

Reuters