LOS ANGELES >> Floyd Mayweather Jr. began promoting his final fight by doing what he does best.

Fending off every attack with defense that looks effortless.

The pound-for-pound king of boxing insisted Andre Berto is a worthy opponent for his grand farewell when they made a joint appearance in downtown Los Angeles on Thursday, just over five weeks before they’ll meet at the MGM Grand Garden Arena in Las Vegas on Sept. 12.

"I don’t know why, but I’m just training a lot harder for this fight," Mayweather said. "I just want to go out with a bang, I guess."

Mayweather also repeated his declaration that this is the last bout of his career, although he grinned broadly while saying it. Mayweather previously retired in 2008, but it barely lasted a year.

"They say they’ll offer me a lot of money" to keep fighting, Mayweather said. "I have a lot of money. I’m OK. … It’s the end of my (Showtime) deal. I’m an old man now. I’m 38."

Three months after Mayweather (48-0, 26 KOs) dominated Manny Pacquiao in the most lucrative fight in boxing history, the welterweight champion was in a noticeably upbeat mood as he began the difficult business of promoting his next bout with Berto. The former 147-pound champion has lost three of his last six fights, including two losses to fighters who have already lost to Mayweather.

Mayweather acknowledged the backlash from his choice of Berto (30-3, 23 KOs), but cited the challenger’s aggressive style and his own determination as reasons to buy the fight. Mayweather, who has knocked out one opponent in nine bouts since 2007, even promised an exciting fight to fans who still feel short-changed by his technical display against Pacquiao in May.

"With the way he comes, and with the way I’m going to come, if it’s not a knockout, there’s going to be some knockdowns," Mayweather said. "A lot. And there’s going to be blood. There’s going to be knockdowns, and there’s going to be blood. A lot."

Mayweather’s last bout raised his profile outside boxing to new heights, but also exposed him to unprecedentedly widespread criticism, with his legal history of violence against women spotlighted throughout the run-up to his win.

Contrasting sharply with the contemplative persona he employed in May, Mayweather was playful in his return to public appearances. He even commandeered a selfie stick to take a photo on the dais during the news conference.

Mayweather only turned serious Thursday when facing off with Berto for a photo opportunity. The fighters stared motionlessly into each other’s eyes for an uncomfortably long period of time.

Berto and Mayweather have been casual friends for several years, yet Berto didn’t disagree with the assertion that Mayweather is a boring fighter. Berto saw the fan reaction to Mayweather’s bloodless brilliance against Pacquiao, and the challenger promised to make Mayweather work harder.

"I know how people feel (after the Pacquiao fight), and it’s my luck that I come right behind that," Berto said. "If I would have paid that type of money, I probably would have felt some type of way, too."

Mayweather laughed off Berto’s first verbal jabs.

"Well, he’s fighting a boring fighter, but he’s getting his biggest payday," Mayweather said.

Mayweather also defended his choice to put the fight on pay-per-view, asking an expected $75 to watch in high definition. Mayweather Promotions CEO Leonard Ellerbe said that putting the fight on free CBS was "obviously something that we explored," but the promotion didn’t have enough time to put together such a deal before Sept. 12.

The bout will complete Mayweather’s six-fight contract with Showtime, and he has repeatedly said he will walk away afterward. Yet Mayweather will face many temptations to continue, including the chance to surpass Rocky Marciano’s 49-0 record as the headliner in May at the opening of MGM Resorts’ $350 million arena on the Las Vegas Strip.

Mayweather previously quit his sport in June 2008, saying he had "decided to permanently retire from boxing" in his news release.

"We know that wasn’t nothing but a vacation," Mayweather said with a laugh.

But even if he was returning to the boxing milieu for the final time, Mayweather projected an aura of calm before heading across the street to watch the WNBA’s Los Angeles Sparks.

"They always tune in," Mayweather said of the haters. "They say one thing, but they do another."