Even at age 43, Nonito Donaire remains somebody not to be messed with.

And this is the exact reason why his Japanese opponent Riku Masuda, who is 15 years younger, is not taking him lightly as they square off in a World Boxing Association bantamweight title eliminator on Sunday in Yokohama.

In fact, during the official weighin, Donaire showed off with his rock-solid abdomen, proof that the Filipino-American four-division champion is determined to keep himself relevant.

Donaire came in at 117 pounds, the same as Masuda.

In his last fight last December in Tokyo, Donaire almost became a world champion when he carved out a split decision but not after giving the much younger champion a big scare in the first half of the bout at the Ryogoku Sumo Arena.

Armed with a 43-9-0 win-loss-draw card with 28 knockouts, Donaire is dead-serious in becoming a world champion and joins an elite list of fighters who became champion again in their 40s.

But Masuda, though relatively lacking in experience, has the tools and the mindset to mess up that bold bid of Donaire.

He enters the ring with a 9-1-0 slate with eight knockouts.

Donaire is regarded as the best Filipino fighter ever next to Manny Pacquiao.

He has won world titles at flyweight, bantam, super-bantam and feather.