Washington, United States-Andre Berto said  he is blocking out the hype for his welterweight world title bout against Floyd Mayweather and believes the pound-for-pound king is no longer the fighter he was.

He may have lost three of his last six fights and be a massive underdog but Berto, 31, a two-time former champion, says he is in the best shape of his life, as he attempts to become the first man to beat his fellow American.

Mayweather says he intends to hang up his gloves after the September 12 showdown in Las Vegas and he infuriated boxing fans in handpicking Berto when there were more than half a dozen more attractive -- but more dangerous -- fighters in the field.

Mayweather will match the 49-0 record of legend Rocky Marciano if -- as expected -- he defeats Berto.

But Berto (30-3, 23 KOs) is laughing off all the flak, talk of records and his opponent's propensity for showing off his staggering wealth to the world on social media and on television.

"I don't get caught up in all that. I don't get caught up in all the hoopla and all the crazy shit," Berto told reporters in a conference call.

"I don't want to get caught up and caught off guard... I'm on a mission."

He was similarly dismissive about people eager to point out those three defeats, one of which came when he had a shoulder injury that required surgery, leaving him fearing for his career. 

"I never really care too much about people hating on my situation. I've had haters since I was at high school, when I was stealing their girlfriends and all that," said Berto, who was born in the United States but identifies heavily with Haiti, where his family are from.

"If you are not doing something right, they are not going to hate on you. I don't care too much about criticism." 

- Berto sees Mayweather 'slippage' - 

Berto says he has the speed, power and hunger to spring a major surprise at the MGM Grand and believes Mayweather (48-0, 26 KOs) is starting to feel the heat at age 38.

Experts say Mayweather, a master of defense, has become slightly easier to hit in recent fights -- though he still had more than enough to put Manny Pacquiao to the sword in his last bout.

Berto agrees he sees some chinks in Mayweather's armor.

"He's definitely showing some slippage throughout the years. I thought the last couple of fights.

"But he's one of those guys who doesn't abuse his body and he's always respected the sport.

"If you respect the sport, the sport is going to respect you."

He added: "But you can be as sharp as you want, but it only takes one slip moment for somebody to capitalize on it."

And put to him that he is the rank outsider, a relaxed-sounding Berto alluded to troubled times in his past, saying: "I don't know the odds and I have not looked at the odds.

"I don't take time out of my day to sit down and look at the odds and see what other people think."

He added: "If that was the case, I should not have made it out of where I came from because those odds were slimmer than none. If I'd looked at those odds I'd be sitting... probably trying to figure it out on the streets, smoking weed.

"Where I come from, there was a lot worse than anything in that ring. Where my people come from, there's a lot worse than anything in that ring."