Manny Pacquiao turned a lot of heads when he won by knockout for the first time in nine years to claim the World Boxing Association "regular" welterweight title, and made a proclamation.

"This is it. I am back in boxing," the 39-year-old Pacquiao said after his seventh-round knockout of Lucas Matthysse on Sunday. "It's just a matter of time. You have to rest and get it back and that's what I did."

It was Pacquiao's first bout since a contentious loss last year to Jeff Horn in Australia, and since parting ways with longtime trainer Freddie Roach to work with Buboy Fernandez.

Pacquiao (60-7-2, 39 KOs) knocked Matthysse down in the third and fifth rounds. He knocked the Argentine down again in the seventh, and Matthysse spat out his mouthpiece, causing a frenzy among Pacquiao fans in the stadium as the contest was waved off.

And what was going through Pacquiao's head during this contest - he felt Matthysse was fighting in a scared manner and appeared to be hurting from every landed punch.

"I am not boasting but ... I think he's hurting from my punches," said Pacquiao, who hadn't won by knockout since 2009.

"Every punch that I throw, he's hurt. I think he's scared of my punches."

Matthysse, who won 36 out of 39 matches by knockout, said Pacquiao was a "great fighter, a great legend," adding that he planned to take a break.

Philippines President Rodrigo Duterte and Malaysian Prime Minister Mahathir Mohamad attended the fight, the biggest in the country since the 1975 heavyweight clash between Muhammad Ali and Australian Joe Bugner.

"I would like to congratulate Senator Manny Pacquiao for giving us pride and bringing the Filipino nation together once more," said Duterte, who flew to Malaysia for the bout ahead of an official visit.

Duterte said in a statement that Pacquiao has proven himself again as "one of the greatest boxers of all time" and that the win will cement his legacy in the sport.