As coach Tetsuya Yamaguchi (山口 鉄也, Yamaguchi Tetsuya, born November 11, 1983 in Yokohama, Kanagawa, Japan) is a former Japanese Nippon Professional Baseball player with the Yomiuri Giants in Japan's Central League.

He was playing in the Missoula Osprey (Advanced Rookie level under Arizona Diamondbacks) before he was drafted by the Giants in 2005.

He was awarded as Central League's Rookie of the Year in 2008, beating his teammate and another award favorite Hayato Sakamoto, by finishing with an 11-2 with 67 appearances, the most wins of all-time in Giants' relief pitchers.

On October 5, 2018, he announced retirement after the season.

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