John Ruiz has reconsidered his decision to retire and wants to become a world heavyweight champion for a third time.
Ruiz announced his retirement immediately after losing his WBA belt to James Toney on 30 April.
But 'The Quiet Man' said: "If they want me out of the sport, they're going to have to knock me out. I'm back.
"I want my WBA belt back. I still feel that I'm the best heavyweight in the world and I'm willing to fight any of the top heavyweights."
Ruiz was easily outpointed by former middleweight world champion Toney at New York's Madison Square Garden.