Victor Ortiz is out to prove Floyd Mayweather is "done" as a fighter when they meet in Las Vegas on 17 September.
Ortiz, 24, defends his WBC title against fellow American Mayweather, a five-weight world champion who is unbeaten in 41 fights.
"He's way overdue a defeat," said Ortiz, who claimed the belt with a points defeat of Andre Berto in April.
"Floyd who? On 17 September, one man's career ends and another man's career begins - he's done."
Despite Ortiz's bold pronouncements, the 34-year-old Mayweather will be a heavy favourite going into the fight, which is his first since he beat Shane Mosley last May.
Victory over southpaw Ortiz, who has lost two and drawn two of his 33 professional encounters, would revive talk of a potential blockbuster match with Philippine great Manny Pacquiao.
Mayweather and Pacquiao were due to meet last year before a row over drug testing scuppered the bout.
Britain's WBA and IBF light-welterweight champion, Amir Khan, has also been calling Mayweather out, although Khan's next opponent is more likely to be faded Mexican legend Erik Morales, who fights Lucas Martin Matthysse for the vacant WBC belt on the undercard of Mayweather-Ortiz.
However, Ortiz is confident of throwing a spanner in the works at the MGM Grand Garden Arena.
"He [Mayweather] can't adjust to me," said Ortiz. "He's not supposed to be here, he's definitely not supposed to be a 147-pounder and I'm going to see to it he knows that.
"Floyd is a good fighter, but I've always thought he wasn't great. I'm not impressed and I'm not going to hold any respect for him like those other 41 he's fought."