One of the biggest upsets in boxing history just went down when Andy Ruiz Jr. beat Anthony Joshua ... and Steve Rolls says he's about to follow suit ... telling TMZ Sports he's gonna beat GGG!!
Gennady Golovkin is coming off his first career loss at the hands of Canelo Alvarez ... and is a MASSIVE favorite in this weekend's fight at MSG ... but Rolls says he's gonna hand him a second-straight L.
"As soon as I got the call for this fight and I accepted this fight, had every intention in the world to do what happened last week in that fight this fight coming up," Rolls says.
Rolls is definitely no scrub -- he's undefeated at 19-0 and won the vacant USBA middleweight title in his last fight in December.
"I look forward to shocking the world once again ... I think it will be one of the greatest upsets in boxing history."
The fight goes down in New York ... and is available to watch on DAZN.