Pound-for-pound king Floyd Mayweather Jnr ran his perfect record to 46-0 with a 12-round majority decision over Marcos Maidana in their welterweight world title unification bout.
The 37-year-old American Mayweather retained his World Boxing Council world title and took Maidana's World Boxing Association belt, winning the fight on two of the three judges' scorecards, with the third scoring it a draw.
"It was a tough competitive fight tonight," Mayweather said.
"I stayed in there. I wanted to stand and fight. I could have made this fight easy, but it would have been boring."
Mayweather, who was an 11-1 favourite heading into the fight, received scores of 117-111 and 116-112. The third judge scored it 114-114.
Argentina's hard-punching Maidana, 30, who defending his title for the first time, suffered the fourth loss of his career to go with 35 victories.
Maidana was upset with the way the fight was scored, saying he was "robbed" of the decision.
"I definitely think I won," Maidana said.
"He didn't fight like a man. Nobody has ever attacked him the way I did. And nobody has ever landed punches the way I did."
Most of Mayweather's recent fights have been boring tactical affairs, but Maidana's relentless pursuit of the unbeaten champion and willingness to go toe-to-toe made for an exciting bout.
Many in the crowd also thought Maidana had done enough to win the fight and his supporters booed the decision, then tried to drown out Mayweather's post-fight interview.
"After the headbutt [in the fourth round that cut him] I couldn't see for two rounds. But that is what champions do, they adjust," Mayweather said.
Both fighters later resorted to some questionable moves.
Maidana hit Mayweather with a low blow in the eighth. Later in the round Mayweather tried to surprise Maidana with a ghost punch, while the Argentinian was trying to communicate with the referee about something the American was doing that he did not like.
In the 11th, Maidana pushed Mayweather partly through the ropes then threw more punches while the American was leaning over backwards almost falling out of the ring.
Maidana landed 221 punches - more than any fighter against Mayweather in his last 38 fights.
Mayweather praised Maidana for taking the fight to him, but blamed the Argentinian for the rough stuff.
"I thought I was the boxer and he was the WWE wrestler," Mayweather said. "It was the toughest wrestling match I have ever had. Miguel Cotto was a tougher fighter and he hit harder."
But asked if he would give Maidana the rematch that the Argentinian says he wants, Mayweather replied, "Let's do it again".
Some see Mayweather's inability to dominate like he has done in the past as a sign Father Time might be catching up.
"He is getting older and showing a lot of mistakes that he didn't used to do. It is a great time to catch him," said British boxer Amir Khan who beat Luis Collazo on the undercard and is also hoping to land a title bout against Mayweather.
Several people were injured and taken out of the MGM on stretchers to the hospital after hundreds of fans stampeded out of the front doors of the Grand Garden arena immediately following the fight.