Mayweather did enough to maintain his unbeaten record, but it was far from a convincing performance in Las Vegas on Saturday night.

The judges scored Mayweather 116-111, 115-111 and 115-111.

But the decision was booed by the crowd and Castillo felt he had been robbed

The Mexican said: "I thought I won the fight. Unfortunately the judges didn't agree with me.

He said he thought Mayweather's local ties and marketability for future fights played greater roles in the verdict than his boxing display.

"They favored him because of that," Castillo said. "Judges should be impartial. They weren't. But I have a guarantee that we will have a rematch.

"(Promoter Bob Arum) didn't give me a guarantee but he said most likely we will have a rematch."

Mayweather said: "Tough fight. Tough guy. I'll fight him again."

The unbeaten Mayweather was moving up after successfully defending his 130-pound (58.5kg) title eight times and was a 7-2 favourite despite never having fought as a lightweight.

He won by using his jab effectively and staying away from Castillo for much of the fight, tactics that did nothing to endear himself to the crowd.

Mayweather (28-0) showed little of the power that had secured him 20 knockouts in his previous 27 fights.

That was partly because he was on the move most of the fight trying to win rounds by using his left jab and some occasional counter punching.

As Mayweather held his world title belt and stood on the ropes, fans booed and some even threw trash at him.

"I was wearing the guy down. I wasn't worried at all," Mayweather said.

"He was a good fighter. It was a good fight. It was a good experience. I wanted to fight the best. I fought the fight I wanted to fight. I'm looking forward to moving on."

Punishment

Castillo earned respect in defeat. Mayweather blamed a huge disadvantage in punches thrown on a shoulder injury suffered in training but unmentioned until after the fight.

"It was tough," Mayweather said. "My left wasn't as strong as I wanted it to be. I beat this guy with a messed up arm. My arm was messed up. But I don't have no excuses. (I won) with one arm."

Castillo landed 46 more punches and connected on a higher percentage, nearly three times as many total punches.

Only in jabs did Mayweather own any edge as Castillo inflicted more punishment than he took.

"At one time I felt him. But I hurt him a lot more," Castillo said. "He felt my punches. A lot of times he stood there and tried to trade with me. But my punches were too hard."