Former world champion Andre Berto says kickboxing sensation Tenshin Nasukawa has the opportunity to ruin Floyd Mayweather's legacy forever, and all he's gotta do is go in there and beat Floyd.

Mayweather is 50-0 and has beaten everyone from Canelo Alvarez to Oscar De La Hoya ... but, according to Berto, that all goes out the window if he loses his December 31 exhibition match with Nasukawa in Japan.

The contest is scheduled for three rounds and Nasukawa is not allowed  to kick, with both fighters competing under traditional boxing rules. There will not be any judges, and no winner will be announced, by there will not be anything stopping the fighters from attempting a knockout victory.

Nasukawa has vowed to knock Mayweather out.

If he's able to make good on his promise, Berto certainly feels Mayweather's legacy will get cracked.

"It would tarnish it for sure. You cant be 50-0 fighting some of the best fighters in the world then get knocked out by some kid," Berto told TMZ Sports.

Berto faced off against Mayweather back in 2015, and says the chances that Tenshin gets the upset in Tokyo are SLIM ... but sees one strategy he can use to pull off the upset.

"Go in there and try to knock his head off, and if you throw a kick ... f*** it," Berto said.

Mayweather announced his retirement in August 2017, after stopped UFC superstar Conor McGregor in the tenth round at the T-Mobile Arena in Las Vegas. There is talk that Mayweather could return to the ring in 2019, possibly in a rematch with Manny Pacquiao - if he beats Adrien Broner on January 19th at the MGM Grand.