Nonito Donaire landed some vicious shots once he switched from southpaw to orthodox in the fifth round Saturday in San Juan, Puerto Rico. Photo / Tom Casino-Showtime

Nonito Donaire learned on Saturday night that he has a lot of work to do to become a proficient fighter from the southpaw stance. Unfortunately for his opponent, he learned it mid-fight.

Donaire abandoned his left-handed experiment against Hernan Marquez in the fifth round, thereafter dominating the small, but tough Mexican before finally stopping him at 2:59 of the eighth round on the Juan Manuel Lopez-Bernabe Concepcion undercard Saturday in San Juan, Puerto Rico.

THE RING’s No. 4 fighter pound for pound was fighting for the final time at junior bantamweight. He plans to move up to 118, where he hopes to meet No. 1-rated bantamweight Fernando Montiel.

And he doesn't plan to permanently abandon the southpaw stance.

“I wanted more rounds,” Donaire said immediately after the fight. “I knew I could figure him out right away from the right-handed stance. But I was confident (in fighting lefty) and everybody on my team believed in me. I may have taken a little bit of a beating for it, but I feel that I needed that.

“I will continue to work on my left-handed stance. I feel so much more powerful. Soon you will see Sweet Pea (Pernell Whitaker) or Marvelous Marvin Hagler from me. That’s what I want to do.”

Donaire (24-1, 16 knockouts) had remained at 115 only because he hoped to engage Vic Darchinyan in a rematch of their 2007 fight, which ended with Darchinyan knocked out. However, the sides couldn’t come to terms and it seems both are moving on.

Donaire, relatively tall at 5-foot-6, also said he has had difficulty making the junior bantamweight limit because his body is growing as he ages – although you wouldn’t know it by watching him Saturday.

Marquez (27-2, 20 KOS) is a solid boxer with a good chin and a southpaw himself. It was Donaire’s decision to challenge himself by fighting left-handed, though, that kept the 5-2 Mexican in the fight through four-plus rounds.

Once Donaire switched to his natural orthodox stance, the beautiful boxer within him emerged and Marquez was finished. Donaire knocked Marquez down in the fifth and then slowly broke him down with the hard, accurate punches for which he is known.

The end came when Donaire landed a perfect left uppercut that sent Marquez sprawling to the canvas in the final seconds of the eighth round. He was able to get up and could’ve finished the round but his corner, sensing the inevitable, saved him from a beating by stopping the fight.

Donaire was just too tall, two quick and too powerful for Marquez, which has been the fate of many of the Filipino-born Californian’s overmatched opponents.

Hopefully, he’ll have better luck at finding big-name opponents at 118 and beyond than he did at 115. Then we’ll see how how he’ll fare when he doesn’t have quite the same physical advantages.

Marquez undoubtedly would predict that Donaire will do just fine.