Making his first defence, Yamanka of Japan weathered an early storm from the veteran, a former two-weight world champion.

Yamanaka opened up a cut over the right eye of his opponent in the fifth round and dominated thereafter, rocking Darchinyan in the 11th with a big left hand.

The judges had the fight 117-111 and 116-112 twice.

"It pretty much went according to plan," said Yamanaka, who moves to 16-0-2. "I was able to land some punches and take the momentum from his as the bout went on."

Also on the Tokyo bill, Japan's Takahiro Aoh defended his WBC super-featherweight title with a unanimous decision over Thailand's Terdsak Jandaeng.

"This was a great way to celebrate my birthday," said Aoh, who turned 28. "I was very motivated and determined to make it a great day."