Justin Gaethje Says James Vick KO Was 'Most Terrifying Night Of His Life'

UFC knockout artist Justin Gaethje says his violent beatdown of James Vick this weekend felt "amazing" for him -- but for the other guy?

"It was the most terrifying night of that man's life."

Yeah, he ain't lyin'.

Gaethje joined the guys on the "TMZ Sports" TV show (airs weeknights on FS1) -- and said it was actually a "scary moment" for Vick ... 'cause getting KO'd is no joke.

"I've been there man. It's scary actually. His brain ... it's like a reset button on your forehead. It's like someone presses it every 30 seconds."

Gaethje was the #7 ranked lightweight fighter in the UFC going into the fight. Vick was ranked #10. The fight ended 90 seconds into the first round when Justin threw a crushing overhand right.

Gaethje says he actually felt bad for Vick -- a little bit -- but in the moment he was focusing on his accomplishment and "it feels f**king amazing."

Despite all the trash talk before the fight, Justin says there's no hard feelings anymore -- because it's way easier to squash beef after you almost knock your opponent's head off.

"It's a painful way to go out."