Ngannou clashes with Ferreira at heated weigh-in
Francis Ngannou weighed in five pounds lighter than Renan Ferreira for their heavyweight bout at PFL Super Fights in Riyadh, Saudi Arabia on Saturday.
Ngannou weighed 256lb (18st 5lb) compared to Ferreira's 261lb (18st 10lb) for the bout the organisation is dubbing the 'Battle of the Giants'.
The towering heavyweights had to be separated by security during their final face-off, as Ngannou put his forehead against his rival's.
Calm was quickly restored and Ngannou said afterwards: "I will let him know who is the king."
Cameroon's Ngannou, 38, is competing in mixed martial arts for the first time in nearly three years, after leaving the UFC 22 months ago.
Since then he has boxed against Britons Tyson Fury and Anthony Joshua, becoming one of the biggest combat sports stars in the world.
Brazil's Ferreira, 34, won the PFL annual global league in 2023 and is on a four-fight winning streak.
In Ngannou's last MMA fight, he defended his UFC heavyweight title against Ciryl Gane in 2021, before leaving the organisation following disagreements over his contract.
He has not lost in the sport since 2018, winning five of his six fights in that period via knockout.
Brazilian featherweight Cris Cyborg weighed in at 144lb (10st 5lb) for her co-main featherweight bout against compatriot Larissa Pacheco, who was slightly lighter.
The main and co-main contests are not official title bouts, but the winners will receive a commemorative PFL Super Fights championship belt.
In a rare move, the only official title fight on the night is third on the card, with American Johnny Eblen at 187lb (13st 3lb) defending his Bellator middleweight belt against Britain's Fabian Edwards, who weighed in fractionally heavier.
The bout is rematch of their 2023 contest, which Eblen won via a third round knockout.
'MMA is Ngannou's bread and butter' - analysis
Paul Battison, BBC Sport
There are not many people who leave the UFC in their prime and go on to better themselves, but Ngannou has never done things by the book.
The Cameroonian was a star in the MMA world when he left the UFC as its heavyweight champion early last year, but he is now a star on the global stage after blockbuster boxing fights with Fury and Joshua.
Fans saw Ngannou take Fury to the brink of defeat on his professional boxing debut, before suffering a devastating knockout by Joshua, but will now get to see him in his more natural habitat.
Mixed martial arts is Ngannou's bread and butter, where he made a name for himself as one of the most devastating punchers in the sport's history.
Ngannou's fearsome stand-up is what first caught the attention of the boxing world, but inside an MMA cage he has many more strings to his bow - proven by his win over Gane in 2022, where he out-wrestled the Frenchman.
He is the favourite against Ferreira, with doubts looming from critics over the standard of challengers in the PFL's heavyweight division.
The biggest win of Ferreira's career came against former Bellator two-division champion Ryan Bader in February, but at 41, the American is in the twilight of his career.
A win over Ngannou, however, would change everything for the Brazilian, thrusting him to the forefront of the global heavyweight MMA landscape.