Nonito Donaire said he was felled by "the hardest punch I've ever been hit with" as bantamweight king Naoya Inoue inflicted a sensational second-round stoppage on the Filipino veteran in Saitama.

The unbeaten Japan boxing great known as "Monster" scored his 20th KO victory in 23 fights to add the WBC title to his WBA and IBF belts in a breathless display of power punching.  

In the pair's first, classic fight three years ago in the same arena, five-weight world champion Donaire fractured a bone in Inoue's face as the Japanese was pushed all the way before scoring a unanimous points victory.

But in Tuesday's rematch the 39-year-old "Filipino Flash" Donaire was demolished like never before in his storied career.

Just seconds before the end of an even first round, Inoue's lightning quick overhand right to Donaire's temple scrambled the senses of the future Hall-of-Famer and sent him sprawling.

"That was the hardest punch I've ever been hit with. I came up completely blank," Donaire said to fans live on his YouTube channel following the defeat.

"When I got hit I didn't even know I got dropped. I didn't see that punch coming at all because I was trying to counter him and got caught. That was pretty much it."

In round two a barrage of precision power punches from Inoue induced wobbly legs in the gutsy Donaire before a second knockdown saw the referee call off the contest.

Donaire had only been stopped once in his previous 48 fights.

He paid tribute to the 29-year-old Inoue, regarded by many as the best pound-for-pound puncher in world boxing.

"He's an amazing fighter and I'm glad I got to share the ring with him," said Donaire.

"Inoue got me really good and I'm glad the referee stopped the fight as I was always going to stand up -- I just don't have any quit in me.

"It is what it is. I'm only grateful that I'm healthy."

Boxing champ Inoue's home burgled during fight: reports

Japanese boxing champion Naoya Inoue reacted with horror on Wednesday after reports said his home had been burgled while he was busy taking down Nonito Donaire at a much-anticipated match.

More than a dozen expensive bags and pieces of jewellery were stolen from the 29-year-old bantamweight champion's home near Tokyo on Tuesday evening, broadcasters TBS and Fuji TV said, citing unnamed police sources.

Local police declined to confirm details of the case when contacted by AFP.

But it appeared the burglars had taken advantage of Inoue's absence as he beat 39-year-old veteran Donaire to add the WBC bantamweight world title to his WBA and IBF belts.

The unbeaten Japan boxing great tweeted in annoyance in an apparent reference to the incident.

"What a disgusting story on an otherwise celebratory day... Everyone be careful!!"

On Tuesday, two men were seen fleeing in a white vehicle after they allegedly prised open the door to Inoue's home, TBS said, adding that police are still pursuing the duo.

Meanwhile, Inoue floored Donaire at the end of the first round and then continued the onslaught into the second at the fight in Saitama, north of Tokyo, winning the title in a breathless display of power punching.

Donaire, nicknamed the "Filipino Flash", had only been stopped once in his previous 48 fights.