Amanda Nunes was pulled from Saturday night's UFC 213 card in Las Vegas due to an undisclosed illness, robbing the event of its headline bout.
Nunes was scheduled to defend her UFC women's bantamweight championship against Valentina Shevchenko, but she reportedly was taken to the hospital Saturday morning after becoming ill, according to mmafighting.com.
UFC president Dana White confirmed Nunes would not fight via a post on Instagram on Saturday.
An interim middleweight title bout betwen top contenders Yoel Romero (13-1) and Robert Whittaker (18-4) is expected to be elevated to the main event for Saturday's card at T-Mobile Arena.
Nunes, the first openly lesbian world champion in combat sports history, has held the title since finishing off Miesha Tate by TKO a year ago this weekend at UFC 200.
The winner of five straight bouts and six of seven, Nunes already defeated Shevchenko at UFC 196 via unanimous decision to earn the shot at Tate's title. From there, she finished Tate in the opening round to claim the title, then waxed Ronda Rousey in less than a minute to retain the belt at UFC 207.
Romero and Whittaker are scheduled for a five-round bout for the middleweight interim championship. The division's champion, Michael Bisping, has not defended the title since October and is out for the foreseeable future because of a knee injury.
Romero, of South Florida by way of Cuba, was a 2000 Olympic wrestling silver medalist. He has finished 11 of his 13 wins and carries an eight-fight win streak into the bout.
Whittaker, a New Zealander now based in Sydney, has blasted his way into contention with seven straight victories, four via knockout.
"I'll do what I need to do," Whittaker said. "I'll go in there and I'll look for a finish and I'm sure he will too, but I'm in there from the first minute to the last minute. Twenty-five minutes is the length of the fight and I'm prepared to go the distance."