And the world's best player apparently tried to use his clout to stay in the tournament, even though his dismissal was automatic under tennis rules.
Djokovic was also been slammed for leaving the tournament without talking to the media.
Following the incident there was a lengthy discussion at the net involving Djokovic and tournament officials, with many veteran observers believing the automatic disqualification should have happened much quicker.
The Daily Mail reported the shocked-looking world number one telling those officials: "She doesn't have to go to the hospital for this.
"You're going to choose a default in this situation? My career, grand slam, centre stage?"
Djokovic did immediately show concern for the lineswoman and went over to check if she was okay after she was struck.
He later posted a message on Instagram apologising for his actions.
"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. 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," he said.
In his most recent post to Twitter, Djokovic asked fans to have sympathy for the linesperson.
"Dear #NoleFam thank you for your positive messages.. Please also remember the linesperson that was hit by the ball last night needs our community's support too. She's done nothing wrong at all. I ask you to stay especially supportive and caring to her during this time," he wrote.
"From these moments, we grow stronger and we rise above. Sharing love with everyone. Europe here I come."
A USTA statement said Djokovic, aiming for an 18th grand slam title, would lose all ranking points, be fined all of his minimum $250,000 tournament prize money, in addition to any other fines for the offence.
Grand slam rules ban intentionally hitting a ball dangerously or recklessly within the court, or in a negligent manner.
The on court discussion involved tournament referee Soeren Friemel and other officials.
Djokovic had already whacked a ball more safely, in apparent frustration, and had just dropped serve to 20th seed Carreno Busta, to fall 6–5 behind.
The lineswoman fell to the ground when struck, holding her neck, and looked in distress.
There are other examples of players being defaulted for similar actions, including in 1995 when Brit Tim Henman hit a ball into the head of a ball girl during a doubles match.
US Open fifth seed Alexander Zverev said: "If it (the ball) would have landed anywhere else – we're talking a few inches – he would have been fine.
"I think the supervisors and all them are just doing their job, but very unlucky for Novak."
The 33-year-old Djokovic was the strong favourite with Rafael Nadal (Covid-19 concerns) and Roger Federer (injury) not playing.
Former US Davis Cup captain Patrick McEnroe is among those who said that Djokovic should have fronted a press conference.
Stuart Fraser, the Times tennis writer, called it "woeful misjudgment by the world number one and leader of the new player association not to front up and apologise for his actions".
