Novak Djokovic has been disqualified from the US Open after hitting a line judge with a ball.

The world number one was mid-match against Pablo Carreño Busta in the fourth round of the tournament, when disaster struck.

The top-seeded player, chasing his 18th Grand Slam title, Djokovic had just lost a game to fall behind 6-5 in the first set.

He hit a ball behind him as the two players moved to the sideline for a change of sides.

The ball struck a female line judge at the back of the court, apparently in her neck, who then dropped to her knees.

After discussions with the match officials, Djokovic was disqualified, and he went to shake hands with Carreno Busta before leaving the court.

Hours later, Djokovic took to social media to apologise.