The bout in Saitama, north of Tokyo, was ruled a draw because it had no judges, with Pacquiao, 45, struggling to dominate Anpo, a 28-year-old mixed martial artist.
The exhibition bout was conducted under strict boxing rules.
Anpo, a former kickboxer, had been threatened before the fight with a $5 million fine if he used any non-boxing moves, such as a back fist or feinting a kick.
Both fighters felt each other out after the opening bell but the 166-cm Pacquiao could not overcome his 180-cm opponent during the three-round bout.
They unleashed a barrage of punches in the final round, with Anpo's onslaught momentarily sending Pacquiao staggering backwards.
"I'm OK," Pacquiao told reporters, adding he was at about "60 percent" of his full capacity.
He said Anpo's height was partly why he wasn't able to knock his opponent out.
The former world champion also said he would "fight in a real fight this year".
Pacquiao retired from professional boxing in 2021 weeks after losing to Cuban Yordenis Ugas in Las Vegas.
He entered politics in 2010 as a congressman before being elected to a more high-profile position in the Senate in 2016.
He gave up the chance to run for re-election in the Senate by making a tilt for the Philippines' presidency in the May 2022 election but lost by a huge margin.
Pacquiao is expected to make his political comeback in the 2025 mid-term elections, telling local reporters in May that he would run for the Senate under President Ferdinand Marcos’s Federal Party of the Philippines.
Anpo said he "didn't really feel like (Pacquiao's) punches were too heavy... They were all within my expectations," he said.
Some social media users marvelled at Anpo's surprising performance.
"What happened to Pacquiao?" one wrote on social media platform X.
"Pacquiao's decline was more than I imagined," another said.
American professional boxer Ryan Garcia, who watched the fight ringside, told reporters that he still saw a vestige of Pacquiao's former ability.
"I didn't think his defence was too good," Garcia said.
