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 squandering three set points when Carreno Busta served at 4-5, 0-40, then suffering a fall. Having dropped serve to trail 5-6, he swatted a ball reasonably hard to the back of the court, inadvertently striking a female line judge in her throat.

With the line judge screaming out in pain as she fell to the...