Undefeated Floyd Mayweather will attempt to match Rocky Marciano's record of 49 unanswered victories against Andre Berto on 12 September.
Former welterweight champion Berto has 30 wins and three losses before his bout with his fellow American.
"Promise you this I'm gonna be as ready as I've ever been. Don't act surprised on 9/12," tweeted Berto, 31.
Britain's Amir Khan had hoped for a September bout with Mayweather, 38, after a win over Chris Algieri in June.