The men's top seed hit a ball in frustration after his serve was broken during the match against Carreno-Busta, the 20th seed from Spain, to put him 6-5 down.

It struck a line judge in the throat and after a discussion officials defaulted the Serbian.

The dramatic sequence of events on day seven at Flushing Meadows in New York started at five games all in the first set when at 0-30 Djokovic fell heavily on his shoulder.

After lengthy treatment the world number one found himself three break points down.

After saving the first break point, Djokovic's serve was broken.

In anger, he hit a ball which struck a line judge, who fell to the floor.

Officials were then involved in a lengthy discussion with Djokovic.

Television broadcasts picked up Djokovic saying "you told me you have a choice" and "you have a game penalty, set penalty, many options" during the exchange with officials which lasted for several minutes.

Djokovic did not give a media conference immediately after the match but later issued a statement on social media in which he said the episode had left him "sad and empty."

"This whole situation has left me really sad and empty," said Djokovic.

"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 apologise to the US Open 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."

In a statement the United States Tennis Association said: "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 US Open tournament referee defaulted Novak Djokovic from the 2020 US Open.

"Because he was defaulted, Djokovic will lose all ranking points earned at the US Open and he will be fined the prize money won at the tournament in addition to any or all fines levied with respect to the offending incident."

Djokovic's default means there will be a new champion in the men's singles competition at Flushing Meadows in a week's time.

During a media conference following the match Carreno-Busta said he did not see the incident because he was looking in the direction of his coach at the time.

He said once he found out what had happened he was "shocked."

He described the incident as "unfortunate" but said "the rules are the rules and the referee and supervisor did the right thing."