Ali's 'Thrilla in Manila' trunks set to sell for $A9m

Auction house Sotheby's said on Thursday (Friday AEDT) the trunks from the October 1975 bout in the Philippines, which was won by Ali and marked a brutal finish to perhaps the greatest trilogy in boxing history, will be up for auction until April 12.

The trunks, which feature a black trim at the waistband and black piping running down the side of each leg, are inscribed by Ali's assistant trainer and corner man Drew "Bundini" Brown and signed by Ali in black Sharpie.

Sotheby's said the trunks were auctioned in 1988 from Bundini's storage locker after his death for about $US1000 ($A1520) and have since made their way through the auction market. They were last auctioned in 2012 for just over $US150,000 ($A227,800).

In the sweltering heat of a Philippine afternoon, the world of boxing witnessed a brutal spectacle as Ali and Frazier fought each other for the third and final time in a clash of wills that etched itself into the annals of sporting history.

In what proved to be a frightfully punishing encounter, Ali retained the heavyweight crown when Frazier's trainer would not allow his fighter to answer the bell for the 15th round.

After the most trying fight of his storied career, Ali said it was the closest he had ever felt to death.

In the first of three bouts between the two boxers, Frazier broke Ali's unbeaten record when he knocked him down with a left hook in the 15th round en route to winning by unanimous decision in March 1971.

Ali got revenge when he outpunched Frazier for a unanimous 12-round decision in January 1974, setting the stage for the "Thrilla in Manila'' nearly two years later.

Ali, whose record-setting boxing career, unprecedented flair for showmanship and controversial stands made him one of the best-known figures of the 20th century, died in June 2016 aged 74 of septic shock due to unspecified natural causes.

Australian Associated Press