Logan Paul Predicts 3rd Round KO, 'I Hate' KSI

Logan Paul says his beef with KSI is 100% real -- telling TMZ Sports he "hates" the YouTube star, and says he's calling his shot in their upcoming boxing match.

Logan and KSI came face-to-face during a heated news conference this past weekend -- things got physical ... and Logan is adamant the hatred ain't just to sell tickets.

"It's real beef," Paul told us on the way out of Catch in West Hollywood ... "I hate the kid."

The two will fight for real in August when they square off in Manchester -- but, for now, it's gonna be a LOT of trash talk.

"I'm fighting a sack of potatoes. KSI, you suck."

Paul says the fight isn't just a big moment for YouTube stars who want to show they're bigger than social media -- it could also be a HUGE moment for the sport of boxing.