Briton Nicola Adams' fight against Alexandra Vlajk in Las Vegas was called off after a problem with her opponent's pre-fight blood test.
Double Olympic champion Adams was due to fight Hungarian Vlajk in a super-flyweight bout on the Saul Alvarez v Gennady Golovkin undercard on Saturday.
Vlajk, 37, has won 11 of her 17 contests and has stopped two opponents since turning professional in 2013.
Adams tweeted she was "devastated" not to be fighting.
The 35-year-old, who said the bout would not go ahead because of a problem with her opponent, added: "Thank you everyone for your support and kind messages."
Adams would have been making her debut in the US and was looking forward to fighting on a high-profile bill.
She told the BBC earlier in the week: "This is something you dream about as a kid. It may never come true and it's nice to think I'm able to achieve my dreams.
"I don't know if we've ever had women on the undercard of such a big event before, or in Vegas. We are definitely changing the game for the better."
Analysis - 'a huge frustration' for Adams
Ade Adedoyin, BBC World Service reporter in Las Vegas
The fight is off because we understand Vlajk failed a medical. I understand it's related to a blood test.
They conducted another test, but didn't have enough time to turn that sample around, so as a precaution the fight was cancelled.
It's a huge frustration for her, with the opportunity to box on such a big bill.