Katie Taylor became the undisputed lightweight champion after coming through the most difficult night of her career against Delfine Persoon.
Persoon was outraged at a majority decision given to Taylor, who could have been handed her first defeat after 10 punishing rounds at Madison Square Garden, New York.
Taylor added Persoon's WBC belt to her IBF, WBA and WBO prizes to complete what she had previously called the biggest achievement of a career which also includes Olympic gold and five amateur world titles.
"I definitely felt that I did enough, especially early. I racked up some rounds," Taylor said afterwards.
"I stand and fight more than I should. I probably should have been more disciplined. I showed heart and definitely did enough to win."
Taylor's hand speed was on show within 30 seconds but she had to be wary of the aggressive Persoon lunging in and trying to create an ugly brawl.
Persoon, who hadn't previously lost for nine years in a 44-fight career, left her mark with a right hand in the second round - a period which was as difficult as any that Taylor has experienced as a pro.
Taylor's tough night got worse when, in the third, she was slightly cut by an accidental clash of heads.
A sensational exchange erupted in the fifth - Taylor beckoned her opponent in and Persoon obliged.
Finally, Taylor's superior speed and skills came to the fore. She became able to measure Persoon's awkward lunges and land counter-punches.
Persoon's left eye was horrendously swollen after seven rounds yet she stormed out of the corner and gave Taylor another nightmarish couple of minutes.
The ninth round was an extraordinary period. Taylor initially landed sharp, classy right hands but Persoon wouldn't fold and struck back with bruising shots of her own that the Irish fighter did well to withstand.
The Belgian felt that she outworked Taylor in a brutal final round before both received a deserved ovation from a thrilled crowd at the final bell.
Two judges scored the fight 96-94 in Taylor's favour and the third called it a draw.