April 25, 2017 1013 GMT
The first knockdown, in the second round, was a combination of a Pacquiao punch and Algieri slipping on the canvas but in the sixth round Algieri tumbled head over heels after a Pacquiao barrage and went down again in the same round from a right hook.
A fierce left cross in the ninth sent Algieri to his back. He beat the count, but dropped to his knees from a follow-up flurry and seemed on the verge of being stopped as Pacquiao unleashed a fusillade of punches against the ropes.
Somehow the American survived that round and despite being knocked down by another left hand in the tenth, made it to the final bell.
Algieri (20-1, 9 KOs), who suffered his first defeat, had earned the shot against Pacquiao by overcoming a swollen right eye and two knockdowns against Russia's Ruslan Provodnikov in his previous outing but he was never remotely in this contest.
Although he circled constantly to his left and attempted to keep Pacquiao (57-5-2, 38 KOs) at a distance with his superior reach, he rarely landed any consequential punches.
“It’s not just his hand speed,” said Algieri afterward of Pacquaio. “He’s a great fighter. He does everything well. I was never hurt, but he did catch me with a big shot.”
Although Pacquiao sometimes struggled to hit Algieri cleanly because of his opponent’s backward motion, the punches he did land took their toll.
“I did my best,” reflected Pacquaio. “Algieri was fast-moving. I’m not surprised that he kept getting up, because that's what he did against Ruslan Provodnikov.”
The win raised once more the prospect of a long-anticipated clash with undefeated American Floyd Mayweather, a fight that Pacquiao said he welcomed.
“I want that fight,” he said. “The fans deserve that fight.”