World No.1 Novak Djokovic has been disqualified from the US Open after hitting a line judge with the ball after a point during his fourth round match against Spain’s Pablo Carreno Busta.

It was a stunning end to Djokovic’s bid for an 18th grand slam title and his 26-0 start to this season.

The top seed had just lost a game to Carreno Busta, to fall behind 6-5 in the first set.

As he walked to the sideline for the changeover, Djokovic smacked a ball behind him.

The ball hit a female line judge, who dropped to her knees at the back of the court.

Tweet from @WordsSlay

After several minutes’ discussion with officials on court, Djokovic walked over to shake hands with Busta, and the default was announced.

There was no question Djokovic did not intend to hit the line judge, and the ball was not hit particularly hard, but it was clear immediately that the woman was hurt, with the Serbian rushing over to check on her condition.

He pleaded his case at length before accepting his fate, collecting his racquets and walking off the court.

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

Djokovic left the tournament grounds without speaking to the media.

A devastated Djokovic apologised to the line judge in a post on Instagram later on Monday.

“This whole situation has left me really sad and empty. I checked on the lines person and the tournament told me that thank God she is feeling OK,” he wrote.

“I‘m extremely sorry to have caused her such stress. So unintended. So wrong.”

The decision will send shockwaves through the sport.

One of the first to react was a scornful Australian star Nick Kyrgios, who has repeatedly locked horns with Djokovic over social media in 2020.

“Swap me for Joker’s incident – ‘accidentally hitting the ball kid in throat. How many years would I be banned for,” he wrote on Twitter.

Tweet from @NickKyrgios

In June, Kyrgios slammed Djokovic’s “stupidity” after it was revealed the world No.1 had tested positive to COVID-19 after hosting a tennis tournament that became the centre of a virus cluster.

Djokovic, whose wife Jelena also caught the virus, apologised for the outbreak that began at the Adria Tour exhibition tournament in the Balkans.

In a statement, the US Tennis Association backed tournament referee Soeren Friemel, saying he had 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 would forfeit ranking points and $US250,000 ($A343,000) 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 – including the 2020 Australian Open – to raise his total to 17. He is 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.

-with AAP