Novak Djokovic has apologised after hitting a line judge with a ball at the US Open on Sunday.The world number one was disqualified from his match against Pablo Carreno Busta after the ball struck the female line judge in the throat."This whole situation has left me really sad and empty," Djokovic said in a statement on Instagram."I checked on the linesperson and the tournament told me that thank God she is feeling OK.

The ugly incident will put a blemish on Djokovic’s resume. He reacted in anger after his serve was broken and this wasn’t the first time that he has lost his cool.

The Serbian has a reputation for letting his frustrations get the best of him and in 2016, he was even warned that his behaviour could one day cost him dearly.

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It came in a press conference after Djokovic had furiously hit the ball into the crowd at the ATP Finals in London.

A journalist asks the tennis superstar whether he is concerned about his continued poor behaviour.

“Does it concern you one day that will cost you dearly? If it hit someone today, is that something you've got to address,” the journalist, Neil McLeman, asks.

Djokovic starts to laugh and says: “You guys are unbelievable. You’re always picking these kind of things.”

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When told that he “keeps doing these things”, he responds: “I keep doing these things? Why don’t I get suspended then?”

Djokovic is then reminded that he could have seriously hurt someone by launching the ball into the crowd, to which he replied: “It could have been, yes. It could have snowed in O2 area as well, but it didn’t.”

Watch the tense affair that has come back to haunt Djokovic below.

GIVEMESPORT’s Scott Wilson says:

Arrogance is the first word that comes to mind when watching Djokovic’s press conference.

He gives no regard to McLeman’s words and clearly didn’t pay attention to his warning.

Four years on and the 33-year-old’s behaviour has now cost him.

Djokovic might have done irreparable damage to his reputation and he only has himself to blame.