Every UFC fighter has an origin story ... and for Demetrious Johnson, his fight career started by the elementary school flagpole -- in a throwdown over WWE wrestling. 

Mighty Mouse joined the guys on the "TMZ Sports" TV show (airs tonight on FS1) -- and was asked if he remembers the first time he was punched in the face. 

DJ recalled it like it was yesterday -- and says he was forced to defend The Rock's honor when another kid said pro wrestling was fake. 

What started ... was the beginning of a beautiful (and violent) career -- a 26-2 record, a 12-fight win streak and a huge match at UFC 216 in Vegas this weekend. 

Check out the entire interview tonight on FS1.