In a stunning end to his 29-match winning streak and bid for an 18th grand slam title, the world No.1 and raging title favourite was sensationally disqualified for the incident after dropping a game in his fourth-round match

As he walked to the Arthur Ashe Stadium sideline for a changeover, trailing Pablo Carreno Busta 6-5 in the first set, Djokovic 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.

"This whole situation has left me really sad and empty," the Serb said in a statement on social media.

"I checked on the lines person and the tournament told me that thank God she is feeling OK.

"I'm extremely sorry to have caused her such stress. So unintended. So wrong. I'm not disclosing her name to respect her privacy.

"As for the disqualification, I need to go back within and work on my disappointment and turn this all into a lesson for my growth and evolution as a player and human being.

"I apologize to the @usopen tournament and everyone associated for my behaviour.

"I'm very grateful to my team and family for being my rock support, and my fans for always being there with me. Thank you and I'm so sorry."

Carreno Busta said he was shocked by the incident.

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 racquet made contact with the ball, and there was concern written on his face as soon as he realised what happened.

Djokovic's mood had soured over the preceding few minutes on Sunday.

In the prior game, he wasted three consecutive break points and after the last Djokovic whacked a ball off a courtside advertising sign.

Then, on the second point of what would become the last game at this year's US Open for Djokovic, he stumbled while chasing a shot and fell to the ground, clutching his left shoulder.

Play was delayed for a few minutes while a trainer checked on him.

On the second point after they resumed, Carreno Busta hit a passing winner to break Djokovic's serve.

That's when Djokovic got himself into trouble.

Australian Associated Press