Amanda Serrano retained her featherweight titles against Danila Ramos in the first women's world title fight held over 12, three-minute rounds since 2007.
The 35-year-old won by unanimous decision after dominating the contest in Orlando to maintain her grip on the WBO, WBA and IBF world featherweight titles.
Women's bouts are usually contested over 10 two-minute rounds, but after going the full 36-minute distance, the seven-weight champion said she "really couldn't feel the difference".
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"In the sixth round, I was like, is it really three minutes or did they go back to two minutes?" she told ESPN after retaining her WBO, WBA and IBF world featherweight titles.
"I felt good. I was in really good shape. I trained really hard for this fight knowing I had to go out there and prove to everybody that women can do whatever they put their minds to."
"This was a historic night," she added. "I hope it's just the beginning, because I know there are a lot of women coming after me. I know the future is bright for us".