Novak Djokovic has apologised for hitting a line judge with a ball at the US Open, saying he is "extremely sorry for creating her such stress".

The Serb took a ball out of his pocket and hit it behind him, striking the female line judge in her throat.

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

"I checked on the linesperson 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."

After a lengthy discussion, Djokovic was defaulted by tournament officials at Flushing Meadows.

He will lose all ranking points earned at the US Open and will the prize money he had won at the tournament. He could also be fined for the incident.

"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," Djokovic's statement on Instagram added.

"I apologise to the US Open 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."

More to follow.