Tennis star NOVAK DJOKOVIC is apologizing after he slammed a tennis ball in frustration, and it nailed a line-judge in the NECK.

It happened at the U.S. Open on Sunday.  In the middle of his fourth-round match against Pablo Carreno Busta, Novak angrily smacked a ball that hit a lineswoman.  It was an accident . . . but it was also unnecessary.

She fell over, and he immediately ran over to check on her.

Novak was disqualified from the match, and he was booted from the tournament.  That’s a big deal because he was the #1 seed.  As for the lineswoman, it sounds like she’ll be okay.

Novak apologized on social media.   Quote, “This whole situation has left me really sad and empty . . . thank God she’s feeling okay.  I’m extremely sorry to have caused her such stress.  So unintended.  So wrong.

“As for the disqualification, I need to turn this all into a lesson for my growth and evolution as a player and human being.  I apologize for my behavior.”