Nick Diaz ended up in the middle of a huge brawl at Hyde Nightclub in Vegas early this morning ... TMZ Sports has learned.
A rep for the club tells us the UFC fighter went to the bathroom around 2:30 AM and accidentally stumbled into another customer ... who started talking smack. We're told Nick apologized and said he didn't want any trouble, but 3 other men then joined the first guy and jumped Nick.
Nick tells us, "I didn't start it. They didn't know who I was."
He tried to defend himself, and got hit hard in the head -- and then the fight spilled out of the bathroom, and out onto the Bellagio's casino floor.