Outspoken welterweight king Floyd Mayweather insists his September 12 showdown with Andre Berto will be his last bout.

"Nobody knows what the future holds, but right now, I'm cool and comfortable and having no second thoughts at all about retirement. I had a good career, I won every world title, but boxing is wear and tear on the body. It's time to hang 'em up. I'm looking forward to working with some up-and-coming kids, going on walks, and taking vacations.''

It's a vision of retirement few who have watched his career can really picture for the ultra-competitive American, who will be trying to equal Rocky Marciano's legendary record of 49-0.

A chance to go to 50-0 could prove irresistible, as could the chance to inaugurate the MGM Grand's new 20,000-seat arena in Las Vegas next year.

But for now the 38-year-old Mayweather says he's glad his success gives him the chance to decide when to quit the ring -- a choice many in the sport are denied.

"Boxing has always been work to me, a job. If I could say anything to a youngster taking up boxing it would be to "be smart and not take a lot of punches. It's called the sweet science for a reason. Hit and not be hit, make a lot of money without taking a lot of punishment. That's what it's about."

That slick defensive style has made Mayweather a target for some critics, as his choice of 40-1 underdog Berto for the bout touted as his last.

"Berto can fight," Mayweather said of the former welterweight world champion, who battled a severe shoulder injury two years ago but returned to knock out Josesito Lopez in March.

"He's hungry. He comes to wage a war," Mayweather said of Berto, who brings a record of 30-3 with 23 knockouts to the fight.