Nonito Donaire's bold bid to become a world champion at 40 will be challenged by a Mexican rival known not just for his durability but for his off-beat style.
The two made weight on the eve of their clash for the vacant World Boxing Council bantamweight title at the T-Mobile Arena in Las Vegas.
Donaire stepped in at 117.25 lbs while Santiago came in at 117.5. The division limit is 118 lbs.
Despite Santiago's advantage in youth, as Donaire is more than 12 years older, oddsmakers have installed the Filipino-American four-division champion the betting favorite.
However, questions remain on whether Donaire would still have the killer moves after a 13-month layoff that was brought about by his savage knockout loss to Naoya Inoue.
The Donaire-Santiago showdown takes place underneath the Errol Spence-Terence Crawford undisputed world welterweight title.
Santiago is familiar to Filipino fights since he battled Jerwin Ancajas to a controversial draw in 2018 in Oakland, California.