Former world champion Bernard Hopkins said Floyd Mayweather has the right to pick the opponent that he wants to face, but he warned the undefeated champion that he should not become too complacent.

Hopkins said in an interview with BoxingScene that Mayweather should not underestimate Andre Berto in their upcoming showdown on Sept. 12 at the MGM Grand Arena in Las Vegas, Nevada.

Hopkins pointed out that Mayweather should not take Berto lightly because the challenger is expected to go all out because he has nothing to lose in their upcoming showdown.

The 50-year-old said that the pressure is on Mayweather's shoulder because the reigning pound-for-pound king wants to preserve his unblemished record, which is currently at 48-0 with 26 knockout victories.

"It ain't what Berto is expected to do or not do, it's what Mayweather do or don't do," Hopkins said. "Mayweather is now in position to be his worst enemy if he's not taking Berto seriously. The burden is not on Berto, he has nothing to lose. But if Floyd Mayweather is not respecting who he's facing, he becomes more vulnerable."

Mayweather, who will defend his World Boxing Council and World Boxing Association welterweight titles in the upcoming showdown, is coming off an impressive unanimous decision victory against Manny Pacquiao during their record-setting fight in May.

Following his victory against Pacquiao, Mayweather announced that his next fight in September could be the last of his illustrious career. His choice of opponent was heavily criticized, as several fighters expressed interest in facing him in what could be his farewell fight.

Former world champion Amir Khan has been lobbying for a shot over the past several months, while Pacquiao has also expressed interest in getting a rematch with Mayweather. But for Hopkins, Mayweather has the right to go out the he wants to.

"I think Floyd Mayweather at 37, 38-years-old has earned the right to go out - if this is his last one - earned the right to go out the way he wants to go out," Hopkins said. "He earned it, beat everyone that was put in front of him. He still has an '0' - something that most fighters don't have who have been in the business as long as he's been."