Djokovic disqualified from US Open after striking line judge

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 failing to take the opening set and 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.

He immediately apologised and stood over her before becoming involved in a lengthy exchange with the tournament referee.

Under the rules of the game there is no option other than a default and despite Djokovic's pleadings that he had not meant to hit the official, he was eventually disqualified.

Djokovic shook hands with a stunned Carreno Busta and trudged 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."