The challenger started really well, using his aggression early and putting a slow starting Tanaka on the back foot frequently with his combinations. It seemed as if Acosta's reputation as a huge puncher had worried Tanaka, who tried to fight off his jab but was often swarmed by the Puerto Rican. It wasn't until round 3 that Tanaka began to find his feet in the bout and he certainly warmed to the task, specifically in the second half of the contest as he began to back up Acosta and land some nasty body blows.
Tanaka's improvement in round 3 continued through much of the bout, as he used his speed, strength and more accurate punching to pick away at the challenger and land some solid shots to head and body, including an uppercut in round 5 that helped drop the challenger, in what was the bouts only knockdown.
Acosta recovered really well from the knockdown, and reapplied his pressure as he began a valiant fight back, but was again damaged to the body by Tanaka, despite some solid flurries from the Puerto Rican. It was back and forth action, but it always seemed like Tanaka's shots were having more of an effect than Acosta's, which were wide and looping and seemed unable to hurt the champion. What also didn't help the challenger was that he began to look incredibly tired, and in rounds 8 and 9 he looked like a man who was seriously wilting.
Despite being clearly tired Acosta refused to back down from the fight, and ended strongly, arguable doing enough to deserve either of the final rounds, both of which were close with Acosta doing everything he could to try and change the bout around. Sadly for his effort he was unable to ever hurt Tanaka, never mind secure the stoppage that he was needing.
At the end of the bout there was no questions over who had won, with Tanaka winning clearly on the scorecards, with scores of 117-110, twice, and 116-111.
Following the bout Tanaka was joined in the ring by WBA champion Ryoichi Taguchi, and it seems clear that a unification bout between the two is something that the camps will begin working on in the near future.