LAS VEGAS (AP) -- Bantamweight champion Amanda Nunes was scratched from her title defense against Valentina Shevchenko after being hospitalized Saturday a few hours before UFC 213.
The UFC cited "illness" - without elaborating - in its statement.
Yoel Romero's interim middleweight title bout against Robert Whittaker becomes the main event of the pay-per-view show at T-Mobile Arena, although the UFC also offered ticket refunds to fans.
After Nunes was hospitalized, UFC President Dana White said 115-pound champion Joanna Jedrzejczyk lobbied him to give her a late-notice fight against the 135-pound Shevchenko.
The Nevada Athletic Commission would never allow such a thing, but Jedrzejczyk and Shevchenko already fought each other three times in muay thai competition during their martial arts careers.
Nunes (14-4), a Brazilian fighting out of Florida, claimed the 135-pound belt from Miesha Tate last July. After Nunes violently stopped Ronda Rousey in December, she was scheduled to make her second title defense in a rematch of her March 2016 victory over Shevchenko (14-2).
Instead, for the third straight year, the UFC has been forced to cancel the main event of its annual International Fight Week show on short notice.
Last July, Jon Jones was removed from his bout against Daniel Cormier three days before UFC 200 for violating the promotion's anti-doping policy. Jones was slapped with a one-year suspension, but his rematch with Cormier is scheduled for July 29 at UFC 214 in Anaheim, California.
The UFC also made multiple changes to its July 2015 main event at UFC 189, eventually showcasing Conor McGregor's victory over Chad Mendes.
UFC 213 lost its likely main event for the first time in late May when men's bantamweight champion Cody Garbrandt decided his back injury wouldn't allow him to prepare for a bout against T.J. Dillashaw.
The cancellation of Nunes' defense pushes Romero and Whittaker into the spotlight, and that's a fortunate break for an showdown between two elite fighters on long winning streaks.
The 40-year-old Romero (12-1), a Cuban living in Miami, has won eight straight fights, including six by stoppage.
Whittaker (19-4), a New Zealand-born Australian, has won seven fights in a row, with back-to-back stoppage victories over top contenders Jacare Souza and Derek Brunson.
The winner will be the interim 185-pound champion while champ Michael Bisping recovers from injury. Bisping has defended his belt just once since claiming it in an upset of Luke Rockhold last July, beating 46-year-old Dan Henderson by decision last October.