'Ultimate Fighter' star Tyler Diamond is giving the blow-by-blow about the violent incident at an MMA event where Nate Diaz slapped him in the face ... and it's nuts.
Diamond -- an undefeated fighter who's killing it on 'TUF' -- says he was at a Combate Americas event in Sacramento on Friday when he saw Nate get into it with his pal, Clay Guida.
Diamond says he got in the middle of the ruckus to try and break it up ... but Nate reached out and slapped him in the face before another guy in Diaz's crew followed up with a punch.