Should Floyd Mayweather Jr end his boxing career on Saturday as planned, his legacy will be underpinned by the extraordinary numbers he has brought to the sport, says former world heavyweight champion Evander Holyfield.

Mayweather has said that his welterweight title defence against fellow American Andre Berto will be his final fight and should he win, as is widely expected, he would match the 49-0 record of former heavyweight great Rocky Marciano.

Should it turn out to be his farewell appearance in the ring, he would bow out as boxing's richest ever prize fighter, having set records in the sport when it comes to annual earnings, pay-per-view buys and gate receipts.

"Mayweather has made more money than anybody else in the game of boxing," Holyfield, who has been crowned world heavyweight champion a record five times, told Reuters.

"He's brought the game up in a way that no other fighter ever has. He took boxing and turned it to a whole other level, whether people believe it or not.

"I don't think we ever thought a welterweight or a middleweight would make more money than a heavyweight but that is what this man has done."

Mayweather, 38, topped the Forbes list of the world's 100 highest-paid celebrities in June with an estimated $300 million in earnings over the previous 12 months.

Holyfield, who had a career record of 44-10-2 with one no contest when he finally retired in June 2014, expressed mixed feelings over Mayweather's legacy as a fighter.

"He is undefeated, which speaks for itself," said the 52-year-old American, who was nicknamed 'The Real Deal' for his own exploits in the ring. "He can fight. He's everything that a lot of people may not give him credit for.

"But he has done things his own way, which has been good but has been bad too. When you talk about being the undisputed champion, he has never been an undisputed champion, but he claimed to be it."

Mayweather is widely regarded as one of the best defensive fighters of all time, and prides himself on his ability to evade punishment with his skilful movement about the ring while making continual adjustments against opponents on the fly.

POLARISING FIGURE

However, he has been a polarising figure throughout his career because of his arrogance and flamboyant showmanship, and has occasionally been criticised for selecting opponents when they are past their prime.

"The rules and regulations should determine what a fighter is, more so than opinion," said Holyfield, who first became undisputed heavyweight champion in 1990 with a three-round stoppage of Buster Douglas.

"Whatever the undisputed champion means is that you are the best if there are three belts and you've got all three belts, then it is undeniable that you are the best fighter in that weight division.

"But if you choose to fight whoever you want to fight and you say 'I'm going to waive this belt', then it takes something away from the game of boxing," he added, in a veiled criticism of Mayweather.

Holyfield, who also dominated the cruiserweight division before moving up to heavyweight, spoke to Reuters after celebrating National Alumni Day earlier this week for the Boys and Girls Clubs of America (BGCA).

The youngest of nine children, he credits the Boys Club in Atlanta for paving the way for his boxing career and teaching him the value of strong character and sportsmanship.

"I don't know if I could have been a champion if it wasn't for the Boys Club," said Holyfield, who is one of 16 million BGCA alumni. "I went there for a whole year and it changed my life, it changed my whole perspective on a lot of things.

"My coach, who was 70 years old, told me that I could be like Muhammad Ali. And I told him, 'I am just eight years old.' But I believed him.

"That sports club had an award for the person who showed good conduct... and I became the person that I am today."