Dricus Du Plessis has taken over two dozen UFC fights and countless others outside of the promotion, but he tells TMZ Sports his toughest match ever will come on Saturday night against Khamzat Chimaev .

The current middleweight champ made no bones about it during a sitdown with Michael Babcock earlier this month ... explaining the UFC 319 main event tilt will not only be "the hardest," but "the biggest fight of my life." Getty

"This is the best opponent -- to date," he said ... adding, "Up until now, this is the best guy. He is the No. 1 contender in the world. He is the best guy without a title in the world."

Du Plessis, though, isn't nervous one bit for the moment ... in fact, he appears to be reveling in it -- as he knows a victory over the undefeated Chimaev could kick s

See Full Page