Manny Pacquiao threw left after left, showing the punch that helped him go unbeaten for more than seven years.
Timothy Bradley kept coming back -- enough times, in fact, that the challenger scored an upset which looked more than possible before the fight started and less and less so until the end.
Bradley won a split decision Saturday night, taking the WBO welterweight belt from Pacquiao and ending the 33-year-old Filipino's 15-fight winning streak.
The bout which started under strange circumstances ended amid gasps and boos at the MGM Grand, setting up what would be a moneymaker of a re-match if it happens.
"Absolutely, yes," Pacquiao said in the ring about fighting Bradley again.
Bradley (29-0) remained unbeaten, winning 115-113 on two scorecards after he took every punch Pacquiao threw, including many after apparently hurting his ankle during an early round.
Pacquiao (54-4-2) had a 115-113 margin on the third scorecard but lost for the first time in four title defenses of the belt.
Bradley was the first fighter to beat one of the sport's biggest stars since Erik Morales won a unanimous decision over Pacquiao on March 19, 2005, at 130 pounds.
The bout, which was being held until Game 7 of the NBA's Eastern Conference finals was over, was suddenly on-deck after an undercard fight ended early when Jorge Arce couldn't continue after taking a punch to his left ear from Jesus Rojas in the second round.
It left Bradley, gloves on, waiting for Pacquiao to get ready and it was close to an hour after the previous bout ended that theirs finally got underway.
Meanwhile, the outcome seemed to leave a potential Pacquiao-Floyd Mayweather Jr. fight in further doubt. Mayweather is currently serving a three-month jail sentence in Las Vegas for domestic violence, but a bout with Pacquiao has long been a wish for fans and observers.
The southpaw Pacquiao landed a hard, straight left with about 15 seconds to go in the first round and two more before the bell -- the start of a pattern that would play out through the fight.
Pacquiao often waited out Bradley, then pounced near the end of a round.
He hit with another left in the final seconds of an active third round that had Bradley wagging his tongue and let loose on a flurry of punches near the end of the fourth, putting Bradley in trouble.
But Bradley sent a message with a right near the end of that round and he was still swinging after being knocked on his heels near the corner in the fifth by a Pacquiao left.
Bradley was backed into a corner again in the sixth, took a hard right to the body, but kept coming back. The eighth, ninth and 10th rounds were closer, even if it looked like Pacquiao won them, and Bradley seemed to close strong, landing a good right late in the 11th and a flurry early in the 12th.
It was the sixth straight Pacquiao fight to go the distance, but none of the others had the shocking outcome of this one. Pacquiao lost for the second time (3-2) against five opponents with no previous losses or draws.
His 15 wins in a row had come with eight knockouts and in four weight classes.