Josesito Lopez stopped Victor Ortiz in the ninth round of their welterweight bout in Los Angeles to claim a surprise victory.
Californian Ortiz was leading on all three scorecards - 86-85, 87-84, 88-83 - but told the referee he was unable to continue after a left hook jolted his jaw.
Lopez also suffered an illegal shot to the back of the head in the fifth that seemed to knock the stuffing out of him before taking victory.
"I told everyone I was going to shock the world," Lopez said afterwards. "Today's my day. Today's my day.
"I'm excited. I had to fight the fight of my life to win. I knew I caught him every round. I might not have thrown the majority of the punches, but the ones I was throwing, I knew I was connecting."
Lopez wasn't even due to fight Ortiz but stepped in as a replacement after Andre Berto failed a drug test.
