Japan's Shingo Kunieda, considered one of the greatest wheelchair tennis players of all time, said Sunday he has hung up his racket.

The 38-year-old Kunieda, a winner of 28 Grand Slam titles and three Paralympic gold medals in the men's singles, retired while at the top of the world rankings.

Shingo Kunieda of Japan plays against Alfie Hewett of Britain in the men's wheelchair singles final at the U.S. Open tennis tournament in New York on Sept. 11, 2022. (Kyodo) ==Kyodo

He won his third Paralympic gold in Tokyo in 2021 and completed his career Grand Slam by winning Wimbledon last year. He also won 22 Grand Slam doubles titles.

"I topped the year-end rankings for the 10th time last year, so I have concluded that I have done enough," Kunieda said on his Twitter account.

"I kept thinking about retiring after my dream came true at the Tokyo Paralympics. I began to feel I didn't have much energy left to play on the tour when I won the long-sought Wimbledon title last year."

Kunieda began using a wheelchair at 9 years old as the result of a spinal tumor.

He took up tennis in his final year of elementary school at the advice of his mother, turning professional in 2009.

"Please forgive me for retiring while being No. 1 in the world," said Kunieda, thanking his team, sponsors and rivals.

"Tough competition allowed me to get this far."

Kunieda spent 582 weeks as men's wheelchair singles world No. 1 during his career, according to the International Tennis Federation.

Kunieda first rose to the top rank after winning the U.S. Open in 2006.

He was unbeaten in men's singles competition at the majors from the 2007 Australian Open through to the 2011 Australian Open, winning 12 Grand Slam titles in succession, including five Australian Opens.

Wimbledon added men's and women's singles wheelchair events to its schedule in 2016 to make all four singles titles available for the first time.

The first Grand Slam tournament of the year was his most successful major -- Kunieda's 2022 victory at Melbourne Park earned him his 11th Australian Open title.