WBA boxing champ Daniel Jacobs says he's sure he's gonna beat GGG in their upcoming fight and all because he's already bested an opponent way tougher than Golovkin ... cancer.
Jacobs was diagnosed with osteosarcoma -- a rare form of bone cancer -- in 2011 and missed nearly 19 months from the sport before resuming his career.
We got Jacobs in L.A. and he told us that fight was the toughest of his life ... and because of that ordeal he has absolutely ZERO fear of GGG, who's knocked out his last 23 opponents.