As he walked to the Arthur Ashe Stadium sideline for a changeover, trailing Pablo Carreno Busta 6-5 in the first set, Djokovic – who was seeded and ranked No. 1 and an overwhelming favourite for the championship – angrily smacked a ball behind him.

The ball flew right at the line judge, who dropped to her knees at the back of the court and reached for her neck.

During a discussion of about 10 minutes near the net involving chair umpire Aurelie Tourte, tournament referee Soeren Friemel and grand slam supervisor Andreas Egli, Djokovic appeared to be pleading his case.

But eventually, he walked over to shake hands with Carreno Busta.

Tourte then announced that Djokovic was defaulted, the tennis equivalent of an ejection.

“I was a little bit in shock, no?” Carreno Busta said at a news conference done via video.

Djokovic left the tournament grounds without speaking to the media.

Asked whether Djokovic should have been allowed to continue to play, Carreno Busta shrugged and replied: “Well, the rules are the rules. … the referee and the supervisor (did) the right thing, but it’s not easy to do it.”

Indeed, the US Tennis Association issued a statement saying that Friemel defaulted Djokovic “in accordance with the grand slam rulebook, following his actions of intentionally hitting a ball dangerously or recklessly within the court or hitting a ball with negligent disregard of the consequences”.

The USTA went on to say Djokovic forfeits the ranking points and $US250,000 in prize money he earned in the tournament – “in addition to any or all fines levied with respect to the offending incident”.

Djokovic had won five of the past seven grand slam tournaments to raise his total to 17, closing in on rivals Roger Federer, who has a men’s-record 20, and Rafael Nadal, who has 19.

With reigning US Open champion Nadal and Federer not in the field, the 33-year-old Djokovic was expected to claim a fourth trophy in New York.

But it all came apart so suddenly on Sunday, even if it was clear that Djokovic did not intend to hit the line judge.

He wasn’t looking in her direction when his racket made contact with the ball, and there was concern written on his face as soon as he realised what happened.

“I think the supervisors and all them are just doing their job, but very unlucky for Novak,” said Alexander Zverev, the No. 5-seeded man, who reached the quarter-finals by winning on Sunday.

“If it would have landed anywhere else – we’re talking a few inches – he would have been fine.”

-AAP