His Nov. 13, 2010, victory over Antonio Margarito gave Pacquiao a record eighth world title. The man who once owned a world flyweight belt was now a super-welterweight champion.
Pacquiao told the Los Angeles Times the Margarito victory is his fourth greatest fight.
The bout, at Cowboys Stadium in Texas, drew more than 50,000 fans and was a supreme test of might versus size.
Contested at a catch-weight limit of 150 pounds, Pacquiao stepped on the scale at 144.6 pounds and Margarito was so famished in making 150 he couldn't end the weigh-in ceremony fast enough, scurrying to his stadium room to dip a banana in a jar of peanut butter.
On fight night, the HBO scale had Margarito weighing 17 pounds more than Pacquiao, 165 to 148.
Margarito was more than two years removed from his infamous beatdown of then-unbeaten welterweight champion Miguel Cotto, a victory that looked a lot more shady when officials found Margarito's gloves were loaded with plaster wraps over his knuckles before a 2009 loss against Shane Mosley at Staples Center.
Still, Margarito's punching power and forward-fighting style was viewed as a threat to the undersized Pacquiao. Was Pacquiao biting off more than he could chew considering he won a lightweight (135 pounds) belt in the summer of 2008?
Margarito was five inches taller, his reach six inches longer.
But Pacquiao's hand and foot speed, along with his creative angles and punching power, provided the answers.
In the fourth round, Pacquiao delivered an uppercut that broke the orbital bone around Margarito's right eye, an eye muscle getting lodged in the fracture and leaving the bigger man with double vision.
That's what it felt like, as the Filipino landed more than 350 power punches on Margarito.
By the 11th round, with the grotesque eye begging for a stoppage, Pacquiao softened, even glancing at referee Laurence Cole to check if he would intervene.
"I am a Mexican. I would never quit," Margarito said.
Margarito was so beat up afterward, he was seen lying on a table with ice packs covering his head and body, telling his manager that Pacquiao was "a special fighter with a God-given talent [who] will make it difficult for any fighter to be victorious against him."
Margarito's eye was so damaged, a specialist was required to repair it. Even after that, he barely passed the scrutiny required by the New York athletic commission to gain a license for a Cotto rematch more than a year later.
The eye area swelled in that fight, Margarito retired on his stool and never fought again.
Pacquiao won every round on one of the judge's scorecards, and while he said he was hurt on the limited occasions Margarito hit him clean – "This was the hardest fight of my career," Pacquiao said afterward -- he won at least 10 of 12 rounds on every scorecard.
"If that had been Mayweather in there tonight, Manny would've killed him," Pacquiao publicist Fred Sternburg said.
Added Pacquiao trainer Freddie Roach: "Mayweather has ducked Margarito in the past. I'm sure this put more fear into him and puts him even further away from wanting to make this fight."