Saturday's scheduled title bout for the vacant World Boxing Organization light-heavyweight crown was postponed Thursday after Russian fighter Maxim Vlasov tested positive for Covid-19.
Vlasov (45-3 with 26 knockouts) was to have faced American Joe Smith Jr. (26-3 with 21 knockouts) at Las Vegas in the 34-year-old Russian's first world title bout.
"I am devastated with the postponement of my world title fight," Vlasov said. "I have been following strict protocols. I have done regular testing with negative results, and I have no symptoms.
"I am well prepared and had an excellent training camp. I look forward to the rescheduling of the fight and the opportunity to display my world class skills."
The remainder of the MGM Grand card will be contested.
Smith, 31, was to have had his second world title fight. He lost a unanimous decision to unbeaten Russian Dmitry Bivol in March 2019 for the World Boxing Association crown.
Another undefeated Russian, Artur Beterbiev, owns the International Boxing Federation and World Boxing Council crowns in the 175-pound (79.4kg) division.