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Tennis world number one Novak Djokovic was disqualified in the fourth round of the U.S. Open 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 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 above the shoulders.

He immediately apologized and stood over her as she appeared in distress on the floor.

Djokovic was clearly making the point that he had not intended to hit the official but under the rules of tennis his fate was already sealed for a violation of the Grand Slam's code of conduct.

The Grand Slam rules state: "Players shall not at any time physically abuse any official, opponent, spectator or other person within the precincts of the tournament site.

A tournament spokesman told Reuters the line judge appears to be okay. Her identity is not known.