Mike Tyson Accidentally Punched UFC Star Fabricio Werdum, 'Almost Broke My Nose'

Things got a little TOO real on Mike Tyson's movie set -- when the 51-year-old accidentally socked UFC star Fabricio Werdum with an actual punch ... and almost shattered his nose!

Mike and Werdum were shooting a fight scene for Jean-Claude Van Damme's new "Kickboxer: Retaliation" when Mike miscalculated ... and REALLY punched Werdum.

"Accident, but he punched me," Werdum tells TMZ Sports ... "almost broke my nose."

Werdum says the punch almost knocked him down -- but he let it slide because Mike was a super nice guy on set.

"It's good 'cause he's Mike Tyson!!"

If ya wanna see Werdum throw hands for real, he's fighting at UFC Fight Night: London this Saturday ... taking on Alexander Volkov in the main event.

Fabricio says he's getting his belt back after the Volkov fight -- but does he think he's gonna take on Stipe Miocic or Daniel Cormier??

We asked. Werdum answered.