World champion Michaela, Britain, Sandy Ryan, has taken the second time to defend the title of her weight in WBO in Las Vegas.
American Mayer, 34, was the winner in an entertainment match between bitter competitors.
After some early rounds, it took control of the middle of the battle with its high production, the selection of the shot and the work of the body.
Ryan, 31, returned, eventually, as some harmful punches fell while the blood was flowing on Mayer's face after the clash of the heads in Fontainebleau.
But with dozens of 97-93, 97-93 and 98-92, California won a unanimous decision 10 rounds.
“I thought it was close. I thought I was initially at first and then faced a battle,” she said.
Mayer cleared Ryan on points in September, a result of the English woman who had a box of red paint before leaving the place in New York.
“Women have become better and better – Sandy and I only put the new tape on what competitive battles should look like,” said Mayer, who scored her record to 21 wins from 23 Nuba.
She called for an undisputed battle against Wilsh women Lauren Price, which bears the next three, next, next titles.
