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Floyd Mayweather will face Andre Berto on 12 September as he attempts to match Rocky Marciano's record of 49 successive victories.

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,"

Britain's Amir Khan had hoped for a September bout with Mayweather, 38, after beating Chris Algieri in June.

Khan thought he was to securing a meeting with the sport's pound-for-pound king after his success over American fighter Algieri and Mayweather's own points defeat of Manny Pacquiao in their lucrative meeting in May.

However Mayweather has instead opted for an opponent who has lost three of his last six fights, with Berto suffering reverses to Victor Ortiz and Robert Guerro - both of whom have also been beaten by Mayweather - as well as Jesus Soto.

"I always bring my A-game and this fight against Andre Berto is no exception," said Mayweather.

"He's a young, strong fighter who is hungry to take down the best - 48 have tried before and on 12 September, I'm going to make it 49."

If he wins, Mayweather's record will equal that of legendary 1950s American heavyweight Marciano, who retired from the sport without suffering a professional defeat.

Mayweather's meeting with Berto will be the final bout in a six-fight deal he agreed with American broadcaster Showtime in February 2013.

Mayweather previously claimed before he retires while his father and trainer Floyd Mayweather Sr claimed in July that his son with which to finish his career.