Koki Niwa

Niwa had a prolific junior career that began on 2008. He reached the quarter-finals at the 2008 India Junior Open in Pune, India, and won the doubles title with his partner, Yuki Hirano. At the 2008 World Junior Championships in Madrid, Spain, he partnered with Kenta Matsudaira to reach the semi-finals. On the following year, Niwa partnered with Asuka Machi to win the 2009 ITTF Cadet Challenge and ITTF Junior Circuit Finals in Tokyo, Japan.

At the 2009 World Junior Table Tennis Championships, Niwa reached the quarterfinals in the men's singles event before he was defeated by Lin Gaoyuan. Niwa qualified for the 2009 World Table Tennis Championships in Yokohama, Japan by defeating Josef Simoncik in the qualification tournament. Niwa reached 64 round before he lost over Germany's Dimitrij Ovtcharov.

At the 2010 Singapore Youth Olympics, he won a gold medal in the Boys' Singles Event, and a gold medal partnering Ayuka Tanioka in the Mixed Team event. Niwa won the U21 title at the 2011 Dortmund Pro Tour German Open by defeating Kim Min-seok. At the 2011 Incheon Pro Tour Korea Open, he defeated Jeoung Young-sik where he obtained his second U21 title that year. At the 2011 Manama World Junior Table Tennis Championships, he won the gold medal in the men's singles event, defeating Lin Gaoyuan of China.

By 2012, the 17-year-old world junior champion was gaining widespread attention. His breakthrough came at the 2012 Asian Olympic Qualification Tournament where he overcame world ranked No. 1 Ma Long and became the inaugural Asian player in the sport to qualify for the Olympics. Partnering with Kenta Matsudaira, Niwa overcame the Chinese pair of Wang Hao and Zhou Yu to win the 2012 World Tour Polish Open doubles title.

Niwa qualified for the 2013 World Table Tennis Championships seeded 15. He reached the fourth round in men's singles event before being defeated by Ma Long. He partnered Kenta Matsudaira in the doubles event and reached the third round before losing to compatriots, Jun Mizutani and Seiya Kishikawa. At the 2013 World Tour Japan Open in Yokohama, the "Niwa-Matsudaira" tandem lost in the finals against compatriots, Jin Ueda and Maharu Yoshimura.

At the 2014 World Team Table Tennis Championships in Tokyo, Niwa won 4 out of his 5 matches, contributing to Team Japan's bronze medal. At the 2014 World Tour Grand Finals in Bangkok, the Niwa/Matsudaira pair lost in the final to Korean pair Cho Eonrae and Seo Hyundeok. Niwa won the 2014 World Tour Russian Open title in the men's singles event, defeating England's Paul Drinkhall in the final. The Niwa/Matsudaira pair competed in the semi-finals and were defeated by the Russian pair Fedor Kuzmin and Grigory Vlasov.

At the 2015 World Table Tennis Championships, Niwa reached the 4th round before being defeated by China's Fan Zhendong. In the doubles event, the Niwa/Matsudaira pair reached the semi-finals, and were defeated by Fan Zhendong and Zhou Yu. This was the last international partnership with Kenta Matsudaira.

At the 2016 World Team Table Tennis Championships in Kuala Lampur, Niwa contributed to Team Japan's success in winning the silver medal. The team lost to Team China in the final. Niwa represented Japan at the 2016 Summer Olympics in the men's singles event. He reached the quarterfinals before being defeated by eventual silver medalist Zhang Jike. In the Men's team event, Niwa contributed to team Japan's first silver medal in the category. However, Niwa expressed discontentment with his performance, not winning any singles matches.

Niwa began the year by winning a bronze medal at the 2017 ITTF Asian Championships in Wuxi, China. In the quarterfinals, he defeated world ranked #3 Xu Xin, but lost to Korea's Jeong Sangeun in the semi-finals. At the 2017 World Table Tennis Championships, Niwa partnered with Maharu Yoshimura to win the bronze medal. The Niwa/Yoshimura pair reached the semi-finals where they lost to Chinese pairing and eventual champions Fan Zhendong and Xu Xin in the men's singles event, he reached the quarter-finals, defeating Dimitrij Ovtcharov in the fourth round, and losing to Fan Zhendong in the quarterfinals. At the 2017 World Tour Japan Open, the Niwa/Yoshimura pair reached the finals of the men's doubles event where they lost to the Chinese pairing of Ma Long and Xu Xin.

In March, Niwa played in the WTT Star Contender event at WTT Doha, but he had an early round of 32 exit to Gustavo Tsuboi.

Niwa played in the singles and team event of the 2020 Summer Olympics. Niwa beat Wang Yang in the round of 32 but lost to Dimitrij Ovtcharov in the round of 16 in the men's singles event at the Tokyo Olympics. In the team event, Japan beat Australia in the round of 16. In the quarterfinals, Niwa upset Mattias Falck to lead to Japan's victory over Sweden. In the semifinals, Japan lost to Germany but won the bronze medal after beating South Korea.

Niwa retired from international competition in November 2022 stating that he wanted to "pass the baton on to the next generation and support them" but will continue to actively compete within Japan.