Challenger Akira Yaegashi won a close-range battle to outpoint fellow Japanese Toshiyuki Igarashi in a World Boxing Council (WBC) flyweight title bout on Monday.
Three judges scored it 115-110, 116-109 and 117-108 in favour of the challenger.
"It's like a dream. A small boxer like me was able to fight in a big class. It was a nice challenge. I'm the strongest in the world," said Yaegashi, who had a 0-4 record against Igarashi when the two were amateur fighters.
"I tried to fight in my style until the end," added Yaegashi, 30, who improved his record to 17 wins, including nine KOs, against three defeats.
Igarashi started cautiously against Yaegashi, who unleashed numerous left jabs and left and right hooks to lead 3-0 after the fourth round.
In the next four rounds both fighters were penalised for butting but each landed hooks to the other's face and opened eyebrow cuts.
Yaegashi still led 3-0 after the eighth round and dominated the following four rounds, landing more accurate hooks and straight blows and almost sending Igarashi to the canvas in the 11th round with a right hook.
It was Igarashi's second defence of the title. He has now 17 wins, including 10 KOs, against two defeats and a draw.