Top seed Novak Djokovic has been disqualified from his US Open fourth-round match after accidentally hitting a ball at a line judge.

The Serbian world number one took a ball out of his pocket and hit it behind him, striking the female line judge in her throat.

After a lengthy discussion, he was defaulted by tournament officials.

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 was the heavy favourite to win the men's singles title at the US Open, which is being played behind closed doors and is the first Grand Slam to take place since the beginning of the coronavirus pandemic.

Going into the match against 20th seed Carreno Busta, Djokovic had not lost a singles match in 2020.

The 33-year-old was aiming for an 18th Grand Slam triumph to move closer to rivals Rafael Nadal and Roger Federer, who are not playing in New York, in the race to finish with the most men's major titles of all-time.

'An open and shut case' - analysis

Russell Fuller, BBC tennis correspondent

If Novak Djokovic hadn't have been defaulted in that situation, can you imagine the outrage? What sort of light would that have shone on tennis neutrality and the decisions that these officials make?

It was an open and shut case to me.

I don't see how you could argue that was not a disqualification. It doesn't matter how hard you hit the ball. I don't think he has any defence at all.

More to follow.