While some pundits believe Tszyu's decision to take on American "Towering Inferno" Sebastian Fundora on 12 days' notice is madness, victory in Las Vegas on Sunday will secure a $US10 million ($A15.3 million) windfall - with even more riches in the offing.

That's the eye-watering figure boxing insiders say the 29-year-old is fighting for at T-Mobile Arena prime time on Saturday night in the USA.

It is not the amount the son of a gun will earn if he beats Fundora, and thus he and Kostya join Leon and Corey Spinks as only the second father-and-son duo to become unified world boxing champions.

Victory would set up a mega-fight with pound-for-pound king Terence Crawford, who last week announced he would exercise his right as WBO welterweight champion to be named mandatory challenger for the winner of Tszyu versus the 197cm Fundora.

Win or lose, AAP has been told that Tszyu (24-0, 17KO) will collect $US10 million ($15.3 million) to enter the ring with Crawford (40-0, 31KO).

That is more than double the $6.56 million golf superstar Cameron Smith pocketed for winning LIV Golf London last year and helping the Crushers to second place in the teams' event on the same day.

Smith also reportedly received between $US100 and $US125 million ($A153-191 million) to defect to the Saudi-backed breakaway tour and the country's best basketball, baseball, soccer players and Formula One drivers can pick up eight-figure sign-on fees.

But no Australian athlete can ever boast of earning $US10 million ($A15.3 million) in one day.

When his original opponent, former world champion Keith Thurman, pulled out of Sunday's non-title pay-per-view blockbuster last week with a biceps injury, Tszyu could have sat back and waited.

Instead he jumped at the chance to take on Fundora, who stands 22cm taller than Tszyu.

The Sydney slayer said "zero" boxers in the world would fight Fundora on such short notice, especially with his WBO world title on the line.

But Tszyu, a self-confessed throwback fighter who is ready for all comers, insists the risk is all Fundora's.

"I'm not the one to fear," Tszyu said.

"He should be fearing me. That's the difference. Nothing stops me, no matter what.

"In my mind, I live in a different era to where we are now. We live in an era of Twitter battles rather than fighters.

"So I hope I can inspire more boxes out there. I saved the show - T-Mobile Arena, all of this stuff.

"Every other fighter that's fighting on the undercard, it's all because of my decision, and it was like that (without hesitation)."

Should Tszyu unify the super-welterweight division, then defend his belts against Crawford, it is understood he won't chase the chance to become an undisputed world champion.

With his legacy already secured, Tszyu craves a showdown with undisputed world super-middleweight champion Saul "Canelo" Alvarez.

Alvarez is arguably the biggest name in boxing and a match-up with the Mexican megastar would assure Tszyu potentially double the money he'd earned fighting Crawford.

Australian Associated Press