Top-seeded Novak Djokovic has been defaulted from his fourth-round match at the US Open after he accidentally hit a line judge with a tennis ball.

It was a stunning end to Djokovic's bid for an 18th Grand Slam title and his 26-0 start to this season.

Djokovic had just lost a game to his opponent, Pablo Carreno Busta, to fall behind 6-5 in the first set.

As he walked to the sideline for the changeover, Djokovic smacked a ball behind him. The ball hit a female line judge, who dropped to her knees at the back of the court in New York City in the United States.

Djokovic 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.

After a discussion of several minutes with officials on court, Djokovic walked over to shake hands with Carreno Busta, and the default was announced.

"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."

More soon...Source: aljazeera.com