American Joseph Diaz lost his International Boxing Federation junior-lightweight crown at Friday's weigh-in, failing to make weight for Saturday's bout against undefeated Shavkatdzhon Rakhimov of Tajikistan.
Diaz, a 28-year-old southpaw, was to have defended the 130-pound (59kg) title for the first time after taking it from compatriot Tevin Farmer in January 2020 by unanimous decision in his most recent fight.
Instead, Diaz lost his championship belt when he tipped the scales at 133.6 pounds and Rakhimov made weight at 129.8 pounds.
Only Rakhimov, a 26-year-old southpaw, can win the title in his US debut in Saturday's bout at Fantasy Springs Resort Casino in Indio, California.
A victory by Diaz, 31-1 with 15 knockouts, over Rakhimov, 15-0 with 12 knockouts, would leave the title vacant.
Rakhimov has stopped rivals inside the distance in 10 of his past 11 fights since April 2016.
"I've been waiting for this moment for a long time. My dream has finally come true so I'm very glad," Rakhimov said of his first major world title bout before the weigh-in.
"The key is to do anything that needs to be done to win. Diaz is going to be in for a big surprise on Saturday. JoJo is a good boxer but he will not be able to handle my power."
Diaz has won five consecutive fights since suffering his only loss in a May 2018 matchup against American Gary Russell Jr. for the World Boxing Council featherweight title.
"I want to show I'm the best at 130 pounds," Diaz said before the weigh-in. "If he thinks he will come in here and overwhelm me with his power -- that's not going to happen."