He then went on a hot streak in 2005, winning the IKUSA 2005 –60 kg Grand Prix in the process, and was given another chance at the AJKF Featherweight title in January 2006 when he beat Genki Yamamoto in a rematch to take the belt.

[1] After amassing an undefeated record of 7-0-2 fighting in the All Japan Kickboxing Federation, Yamamoto was invited to compete in the promotion's Strongest Lightweight Tournament 2004.

This led him to a rematch with Genki Yamamoto for the AJKF Featherweight title on January 4, 2006 at the New Year Kick Festival 2006, where he won a majority decision and became the new Japanese champion.

After being stopped by Muay Thai exponents PKP Rachanon F16 and Wanrop Weerasakreck in his next two outings, he then returned to winning ways with four consecutive victories which earned him a call up to fight on kickboxing's biggest stage, K-1.

The tournament kicked off with the first round and quarter-finals on July 24, 2009 and Yamamoto defeated both Junpei Aotsu and Haruaki Otsuki by decision to advance to the final stage on November 2.

He met Genki Yamamoto for the third time in the semis and an extension round was needed to separate them, after which Masahiro was given the nod to advance to the final where he went up against Yuta Kubo.

[8] Having already established himself as arguably the top kickboxer at 63 kg/138 lb, Yamamoto returned to K-1 as one of the favourites to win the K-1 World MAX 2010 –63 kg Japan Tournament.

[32][33] He participated in La 20ème Nuit des Champions -62 kg/136 lb tournament featuring four of the world's best lightweights in Marseille, France, on November 23, 2013, losing to Yetkin Özkul on points in the semi-finals to exit the competition.