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Eddie Hearn predicted a "generational fight" as Artur Beterbiev and Dmitry Bivol weighed in before their undisputed light-heavyweight bout in Riyadh, Saudi Arabia.
WBO, WBC and IBF champion Beterbiev came in at 12st 7lb, exactly on the limit, with WBA title holder Bivol weighing 12st 6lb.
"This is a generational fight between two ice-cold competitors," said Matchroom's Hearn, who promotes Bivol.
"Bivol already slayed the pound-for-pound king in [Saul 'Canelo'] Alvarez and this is what he wants more than anything.
"I believe Dmitry Bivol will be the undisputed champion tomorrow night."
The unbeaten pair meet in a historic bout on Saturday with all four light-heavyweight belts on the line for the first time.
Roy Jones Jr was the last undisputed champion in the division, earning that status in 1999 and reigning until 2002, but that was during the three-belt era.
A crowded stage saw the three promoters involved in the event - Hearn, Frank Warren and Ben Shalom - jostling for position and the priority middle spot between the two fighters facing off.
Heavyweights Frazer Clarke and Fabio Wardley provide the chief support when they meet in a rematch after they fought to a draw in March.
British heavyweight champion Wardley was a pound lighter than in their previous encounter, tipping the scales at 17st 4lb.
Clarke came in at 19st 5lb - up 5lb from seven months ago - which made him 29lb heavier than his opponent.
Jai Opetaia and Briton Jack Massey exuded confidence when they stepped on to the scales, and both came in at 14st 3lb.
IBF cruiserweight champion Opetaia has yet to taste defeat in 25 fights and has his sights set on unification bouts if he defeats Massey.
"I'm ready to put on a show," Opetaia said. "Everyone wants to be a tough guy but skills pay the bills."
Australia's WBA women's featherweight champion Skye Nicolson and British challenger Raven Chapman both weighed 9st.
Saturday's meeting between Nicolson and Chapman is set to make history as the first women's boxing world title fight in Saudi Arabia.
After a heated exchange during Thursday's news conference, Ben Whittaker tried to draw a reaction from light-heavyweight rival Liam Cameron when they came face-to-face.
Cameron refused to bite, and the pair, who both came in at 12st 7lb, ended up dancing on the stage.
Follow live text commentary of Beterbiev v Bivol and the undercard from 19:00 BST on Saturday on the BBC Sport website and app.