UFC heavyweight champ Stipe Miocic wasn't always a stone cold bad ass -- he's man enough to admit the first time he got punched in the face ... he ran home crying.
Of course, Stipe was only 10 years old at the time ... but Stipe says he's GRATEFUL to the guy who clocked him in the mouth, because it made him into the wrecking machine he is today.
In fact, Stipe has a message to the guy -- "Thank you for making me tougher."
Now, Stipe is one of the most dangerous men on the planet .