An emotional Koji Uehara announces his retirement from baseball on May 20. (Video footage by Kazuhiro Nagashima)

Priding himself as a "mongrel" among the pedigreed players on the Yomiuri Giants, pitcher Koji Uehara overcame his underdog beginnings to shine on the mound in both Nippon Professional Baseball and the major leagues.

Uehara, whose lights-out relief pitching helped the Boston Red Sox win the 2013 World Series, announced his retirement on May 20.

At 44, Uehara was the oldest player in NPB. At an emotional news conference in Tokyo, the right-handed hurler said he was finally calling it quits because he could not physically perform on the mound anymore.

He never made the Giants' roster this season and had difficulty keeping batters off the bases even while pitching on the farm team.

"I wanted to play a while longer, but I am grateful for having played (professional) ball for 21 years," Uehara said.

Uehara attended the Tokai University-affiliated Gyosei High School in Osaka, but his team never reached the prestigious National High School Baseball Tournament, which is held every summer at Koshien Stadium.

He also failed to get into his first choice of universities out of high school, so he waited a year before entering Osaka University of Health and Sport Sciences.

That major bump in the road he experienced when he was 19 was why Uehara chose that uniform number when he was drafted by the Giants in the first round in 1998.

He made an immediate impact his rookie year, posting a 20-4 record as well as recording the lowest ERA and most strikeouts. For that performance, he was named Rookie of the Year as well as the recipient of the Sawamura Award, the Japanese equivalent to the Cy Young Award.

After 10 seasons with the Giants as primarily a starter, Uehara went to the majors, where he pitched for the Baltimore Orioles, Texas Rangers, Red Sox and Chicago Cubs before returning to Japan in 2018 to re-sign with the Giants.

Uehara appeared in 436 games in the majors as mostly a relief pitcher and closer, posting a 22-26 record with 95 saves and a career ERA of 2.66.

In NPB, Uehara pitched in 312 games, retiring with a stellar 112-67 record and 33 saves. His NPB ERA was 3.02.