LOS ANGELES (REUTERS) - Andre Berto will go into his welterweight title fight against Floyd Mayweather in Las Vegas next week as a heavy underdog but also as a man "on a mission". He is bidding to end his fellow American's perfect 48-0 record.

Berto, 31, claims he has never been fitter as he prepares to step into the ring at the MGM Grand Garden Arena on Sept 12.

"I've had a tremendous training camp, a long camp but a tremendous camp," the twice former welterweight world champion Berto told reporters on Tuesday.

"I've never run harder. I'm definitely in the best shape that I've ever been in.

"I've worked with a lot of different sparring partners. Floyd is a very skilled fighter but I think I bring a combination of speed and power to the table.

"I think I have a better athletic ability. I can keep up with Floyd."

Mayweather will put his unbeaten record on the line against Berto, who has lost three of his last six fights and is viewed by some as an easy opponent for their welterweight clash.

Should five-division world champion Mayweather win the WBC and WBA welterweight title bout as expected, in what he has said will be the final fight of his career, he would match the 49-0 record of former heavyweight great Rocky Marciano.