US Open favourite Novak Djokovic has been sensationally thrown out of the tournament after hitting a line judge with a ball.

The top seed and 17-time grand slam champion had just dropped serve to trail Spanish opponent Pablo Carreno Busta 6-5 in the opening set of their fourth-round match on Arthur Ashe Stadium.

He hit a ball behind him in annoyance that hit a female line judge. A lengthy discussion followed between tournament officials and Djokovic as the Serbian argued his case to no avail.