World number one Novak Djokovic was sensationally disqualified in the US Open fourth round on Sunday. The disqualification happened after striking a line judge with a ball during the first set of his match against Spain's Pablo Carreno Busta. Becoming frustrated after squandering three set points, he swatted a ball reasonably hard to the back of the court. The ball then inadvertently struck a female line judge in her throat. A horrified Djokovic ran over to her and apologised, but his fate was soon sealed.