Video of Deontay Wilder Squaring Off with Contender in Hotel Lobby

Deontay Wilder was still ready to swing hours after defending his belt -- getting in the middle of a hotel brawl involving heavyweight contender Dominic Breazeale ... and TMZ Sports got video of the melee.

The chaos erupted in the lobby of the Westin in Birmingham, Alabama ... just across the street from the arena, where hours earlier Wilder scored a 5th round KO of Gerald Washington. In the video, you can see Wilder give someone a huge shove. Breazeale doesn't appear to be the one who received the blow.

The champ mentioned in a post-fight news conference he had beef with Breazeale -- who had fought on the undercard -- and it appears it all came to a head when their entourages met in the lobby.

Cops reportedly broke up the fight after it spilled into the hotel parking lot. Neither Wilder or Breazeale was arrested, but Dominic made it clear on social media ... this feud is just getting started.