Evander Holyfield ain't giving up on Andy Ruiz -- telling TMZ Sports he strongly believes the former champ can win his belts back ... if he busts his ass!
Remember, Holyfield and Ruiz go WAYYYY back -- they famously sparred back in the day when Evander was training for his comeback in 2011.
Ruiz was a nobody way back then ... but Evander has said he could tell Andy had killer potential and wasn't shocked when he beat Anthony Joshua back in June.