I can understand why you disagree but the fact he unintentionally hit the line judge is not a valid argument. The fact remains he hit the ball behind him without looking where the ball was going and he had no reason whatsoever to do it and that is reckless behaviour according to the rules. If a player had done the same thing while in the middle of a rally while trying to get the ball back over the next to their opponent then it would have been irrelevant.

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. The referee, in consultation with the Grand Slam chief of supervisors may declare a default for either a single violation of this code."

And the rulebook states players shall hot the ball: "....dangerously or recklessly within the court, or with negligent disregard of the consequences"

As a result of this incident Djokovic has also been fined $10,000, he will also have to forfeit his $250,000 earnings in the US Open and he will also lose his ranking points earned in the tournament.