Filipino legend Manny Pacquiao rolled back the years to become the oldest welterweight champion in boxing history with a thrilling split-decision victory over WBA champion Keith Thurman on Saturday.

The 40-year-old Pacquiao, 10 years older than the previously undefeated Thurman, delivered a vintage performance before a sell-out MGM Grand Garden Arena crowd of 14,356 which included former welterweight king Floyd Mayweather.

Pacquiao floored Thurman with an electrifying combination in the first round and remained on the front foot throughout a pulsating 12-round duel.

Although the bloodied Thurman rallied in the later rounds to narrow Pacquiao's lead, the judges scored it in the Filipino's favour. Two judges scored it 115-112 for Pacquiao with the third scoring it 114-113 for Thurman.

"It was fun," Pacquiao said afterward. "My opponent is a good fighter and boxer. He was strong."

Thurman had riled Pacquiao in the build-up to the fight by vowing to "crucify" his deeply religious opponent, and later promising to send the Filipino icon into retirement.

Pacquiao, who improved to 62-7-2 with 39 knockouts, was gracious in victory.

"I'm not that kind of boxer who talks a lot; we were just promoting the fight," he said.

"I want to clap my opponent. He's very tough. He can fight. This guy is a warrior."

Pacquiao, who earned an estimated $20 million from Saturday's fight, is now a major player in the competitive welterweight division once again after capturing Thurman's WBA welterweight 'super' champion belt.

The eight-division world champion said he will now return to the Philippines to resume his work as a senator before deciding on his next move inside the ring.

"I think I will fight next year," said Pacquiao, who was due to fly out of Las Vegas for the Philippines shortly after the fight. "I will go back to the Philippines and work and then make a decision."

Thurman meanwhile had few complaints in defeat, embracing Pacquiao after the final bell in recognition of an epic contest.

"I knew it was too close," Thurman said. "He got the knockdown so he had momentum in round one."

"This was a beautiful night of boxing. I wish I had a little bit more output to go toe to toe. I felt like he was getting a little bit tired, but he did have experience in the ring. I would love the rematch."

Official statistics showed Pacquiao landed 195 of 686 punches thrown, while Thurman connected with 210 of 571.

"My numbers just weren't up to par to be victorious tonight," Thurman said. "The fight just flew by. I knew I had to do something big in the later rounds but he was just too good."

Earlier, a pumped-up Pacquiao had danced into the ring to the strains of Rocky movie anthem "Eye of the Tiger" to bring the crowd to its feet.

Deafening chants of "Manny, Manny" rolled around the packed arena as Pacquiao immediately set about building an advantage.

Towards the end of the round, the crowd erupted as Pacquiao sent Thurman crashing to the canvas, uncorking a left to the ribs before detonating a right hand on the American's jaw.

That set the tone for the early rounds, with Pacquiao urging Thurman on before using his superior hand speed and footwork to bamboozle his rival.

A dazzling combination wobbled Thurman again at the end of the second, and Pacquiao delighted the crowd with an Ali shuffle as the round closed.

Although Thurman improved, Pacquiao was able to control the tempo and had Thurman in trouble again in the fifth round with blood pouring from his nose.

Thurman though was in no mood to surrender and he came back strongly, snapping Pacquiao's head back with a right cross-left jab combination in the seventh round.

The younger Thurman appeared to be gaining momentum as the fight drew on, backing up Pacquiao with a clean combination in the eighth.

But again, Pacquiao responded strongly, swarming forward in the ninth round to have Thurman rattled again.

A ferocious Pacquiao body shot in the 10th had Thurman in trouble, again pushing the contest further out of reach.

Pacquiao drew on all of his experience in the 12th, skipping out of range cleverly to keep Thurman at bay and score a famous win.