Rene Santiago produced a stunning upset to claim the WBO junior-lightweight title in a unanimous decision victory over Japan's Shokichi Iwata at the Ryogoku Kokugikan Arena in Tokyo.

The scores read 118-110, 117-111 and 116-112 in favour of the Puerto Rican.

A masterful performance on the road which began with early Iwata (14-2, 11 KOs) dominance, the story of the fight was clear from both fighters' faces and their body language at the conclusion of their fight.

"I want to thank god and thank the country of Japan for having me. I'm ready to bring my belt back home," said Santiago (13-4, 9 KOs) in the ring after.

"I was fully prepared for this bout against Iwata, we had a gameplay which we executed to perfection. I want to take a good rest and figure out what the next step is, but I'd love to come back to Japan."

Round one saw both men feel their way into the fight, with the champion Iwata looking to establish some early dominance by taking the centre of the ring. It was the home fighter who landed first blood by sinking in a swift right uppercut, but Santiago managed his own success with several right hands of his own. Perhaps the opener in the bank for Santiago.

Shibuya's Iwata threw caution to the wind in the second round, walking forward like a man possessed in search of power shots he hoped would end the fight early. As a result, however Santiago had the chance to test his opponent's chin with his own slick offence.

Continuing with his plan to march forward and land at will, Iwata had his first big success in the third by landing a couple of big body shots. And he only continued that attack in the next round, punishing his man with more digs to the Puerto Rican's midriff.

Cracks started to show for Santiago in the fifth.

Iwata only increased his intensity and pressure. Boxing was now out the window; all Iwata wanted to do was get his man out of the ring in swift fashion, and he even had Santiago hurt against the ropes for a moment.

Back to the body went Iwata in the middle rounds. With Santiago holding up to most of the punishment, it was time to try and wear the 32-year-old down. But Santiago wouldn't yield.

Iwata's all-out pressure was order of the day once again in the seventh. The 29-year-old landed a clubbing right hand to begin the round but Santiago, subtly switching southpaw on a couple of occasions, responded in kind with his own accurate flurries.

Despite all of his offensive work up to this point, it was Iwata's face who looked the more swollen of the pair's. And before the eighth, Santiago was told by his corner that four more rounds of this would win him the fight.

The eighth saw some more solid boxing work from Santiago. Re-energised by his corner's words, he looked lighter on his feet, evading and blocking shots from Iwata while he landed potshots back in his direction.

It was fair to say that Santiago found his flow heading into the final third of the fight. Left hooks and uppercuts both landed for the visiting fighter and heading into the ninth, Iwata looked the more tired of the two.

In an attempt to get himself going before round nine, Iwata banged his gloves against his legs twice; the feeling that four important rounds were about to follow.

But it was a lack of patience which was beginning to undo Iwata's early work. Although we barely saw it in the first few rounds, he'd totally neglected the jab beyond round five.

Though Santiago had another positive round in the ninth, Iwata enjoyed success of his own by landing with an overhand-right counter; the kind of shot that usually would see the fight turn on its head. But Santiago finished the round stronger again, landing a swift left hook to keep his opponent honest.

By the 10th, the pattern of the fight had completely settled and the story was clear for judges to begin to ponder. The first half of the fight had Iwata in total control. But heading into the championship rounds it was Santiago who looked the fresher man, taking control of the action from round seven.

Iwata came out for the 11th with a real sense of urgency. His feet quickened up and his hands were beginning to connect with Santiago like they had done in the early rounds. But Santiago finished the round strong again, landing a beautiful combination that ended with a huge right hand which he stopped to admire with a fist pump into the air.

With the fight hanging in the balance, it perhaps looked like the fight was all on the 12th round. It was Iwata's front-foot pressure, perhaps with the knowledge he needed a knockout, versus Santiago's movement and counter-punching, and ultimately it was going to come down what the judges preferred, as is often the case in boxing.

In the end, it was Santiago who had his hand raised. Due to the language barrier, the visitor originally thought Iwata had been given the nod, but upon realising what he'd done, raucous celebrations ensued with his team in the ring.