Irishman Ian Machado Garry suffered the first defeat of his career as Shavkat Rakhmonov edged their welterweight contest by decision at UFC 310 in Las Vegas.
In a gritty contest lacking eye-catching moments, Rakhmonov's advantage in grappling proved to be the difference.
The defeat ended 27-year-old Machado Garry's 15-fight win streak, while Kazakhstan's Rakhmonov earns a title bout against champion Belal Muhammad.
Victory also extended 30-year-old Rakhmonov's own unbeaten record to 19, with the bout marking the first time he has not finished his opponent.
Rakhmonov praised Machado Garry after victory, calling him a "great guy" before calling American Muhammad inside the octagon for a faceoff.
In the main event, Brazil's Alexandre Pantoja defended his flyweight title, submitting Japan's Kai Asakura, 30, in the second round via rear-naked choke.
The victory over UFC debutant Asakura - a former two-time bantamweight champion in Japanese promotion Rizin - was 34-year-old Pantoja's third defence of his belt.
It was also announced during the broadcast that South Africa's Dricus du Plessis will defend his middleweight title against American Sean Strickland at UFC 312 in Sydney, Australia on 9 February.
China's Zhang Weili will put her strawweight belt on the line against American Tatiana Suarez on the same card.
Machado Garry took the fight against Rakhmonov on short notice last month, replacing Muhammad when the champion withdrew with injury.
Rather than make the bout with Rakhmonov an interim title fight, the UFC made it a number one contender bout with Machado Garry adding "everything was on the line".
The Irishman has risen quickly through the UFC ranks since his debut in 2021, winning eight fights but in Rakhmonov was facing a man described as the UFC's 'Boogeyman'.
The Kazakh fighter had stopped all 18 of his previous opponents - until Machado Garry.
The pair showed respect to each other during fight week having trained together in the past, and Machado Garry greeted Rakhmonov with a fist-bump as he entered the octagon.
The first two rounds saw little significant action with the pair battling for position in the clinch against the cage and Rakhmonov landing a couple of eye-catching right hands.
Machado Garry started to find success in the third as he opened up with an array of kicks, before having to change his shorts at the end of the round after sporting a large rip down the back.
Rakhmovov took control in the fourth round as he landed the first takedowns of the fight before delivering some big blows on the ground.
With Machado Garry knowing he likely needed a finish to secure the win, his best moment came in the final round.
After taking Rakhmonov's back, Machado Garry battled for a rear-naked choke on the ground as the crowd roared the Irishman on, but the Kazakh defended well before navigating safely to the end of the contest.
The pair showed further respect, kneeling in front of one another and sharing some words after the bout, before raising each other's arms in the air.