Former major league pitcher Koji Uehara, who won the World Series with the Boston Red Sox in 2013, announced Monday his baseball career is over.

"I want to end my 21-year baseball career today," Uehara, who is under contract with the Yomiuri Giants in Japan, told a press conference in Tokyo. "There is a part of me that wants to play on, but I decided from the beginning (of the season) that this is my last."

The 44-year-old right-hander, who made his professional debut with the Giants in 1999 and rejoined the team in March last year, said he has been unable to put in solid performances this season.

"I thought the first three months of the season were crucial. I was feeling very conflicted because I wasn't called up to the top team and at the same time I was unable to play well at the farm team," he said with tears in his eyes.

(File photo taken in Boston in October 2013 shows Koji Uehara celebrating the Red Sox's World Series victory.)

"I would like to thank everybody who has supported me. I'm very satisfied," he said. "Having made this decision, I need to come around to the fact that I will no longer play in a uniform."

In addition to his World Series title in 2013, Uehara had stints, mostly as a reliever, in the big leagues with the Baltimore Orioles, Texas Rangers and Chicago Cubs.

After rejoining Yomiuri last year, Uehara went 0-5 in 36 games with a 3.63 earned run average, but has not figured in the first team this season. He underwent surgery on his left knee last autumn and was released at season's end, but agreed to a new contract with the team that made him the oldest player with a current NPB deal.

Uehara was the CL's 1999 rookie of the year, and won the Sawamura Award that season as Japan's most impressive starting pitcher, posting 20 wins.

(Pitcher Koji Uehara (L) and his son are pictured during an interview after the Boston Red Sox win the World Series title in October 2013.)

Over a 10-year span, he posted 112 wins, 62 losses and 33 saves with a .301 ERA, before moving to the U.S. as a free agent in 2009.

Uehara spent time with the Orioles and Rangers before finding a more permanent home with the Red Sox from 2013 to 2016.

In 2013, he posted 21 saves and a 1.09 ERA. He was named American League Championship Series Most Valuable Player before going on to help the Red Sox win the World Series.

He pitched in five of the six games for Boston in that year's World Series matchup with the St. Louis Cardinals, including closing the final game of the series by pitching a perfect ninth inning at Fenway Park.

In 2017, Uehara was with the Cubs. He pitched in 49 games, winning three, losing four and saving two with a 3.98 ERA. He did not appear in the postseason and was released after that campaign.

In the major leagues, Uehara has a career record of 22-26, with 95 saves and a 2.66 ERA. He also contributed to the Japanese national team's win at the inaugural World Baseball Classic in 2006 and played at two Olympic Games, winning bronze in 2004 and finishing fourth in 2008.