2018–2019 In 2018, Watanabe reached his first
World Tour final at the Super 100
Russian Open, finishing as the runner-up. After the international circuit resumed following the COVID-19 pandemic, he made his Super 1000 debut at the
2021 All England Open, where he lost in the first round to
Viktor Axelsen. Later that year, he made his
World Championships debut in
Huelva, advancing to the second round. His best result of the 2022 season was a quarterfinal finish at the
Australian Open.
2023 In March, Watanabe won his first World Tour Super 300 title at the
Swiss Open, defeating second seed
Lee Zii Jia in the semi-finals and
Chou Tien-chen in the final. On the World Tour, he won his first Super 500 title at the
Canada Open, defeating compatriot
Kodai Naraoka in the semi-finals and
Alex Lanier in the final. Later in the season, Watanabe advanced to his first Super 750 final at the
Denmark Open, where he defeated
Olympic silver medalist
Kunlavut Vitidsarn before finishing as runner-up to
Anders Antonsen. Watanabe also reached the semi-finals of the
Arctic Open and the
U.S. Open, and the quarter-finals at the
All England Open,
China Open, and
India Open. His notable victories during the season included a win over world number 3
Li Shifeng at the India Open and two wins against Olympic bronze medalist Lee Zii Jia at the China and Arctic Opens. Following these performances, Watanabe reached a career-high world ranking of 11 on 22 October.
2025 Watanabe experienced a challenging start to his 2025 season, suffering early-round exits in several tournaments, including the Super 1000
Malaysia Open and
Indonesia Open. His form improved in July at the Super 750
Japan Open, where he reached the semifinals of the tournament for the first time. En route, he defeated
Chou Tien-chen in the second round and
Weng Hongyang in the quarter-finals. Watanabe credited advice from compatriot Kento Momota, which encouraged him to adopt a "challenger spirit" to overcome his slump. Later in the season, he reached the quarter-finals of the
Arctic Open and
French Open, although he was eliminated in the first round of both
World Championships and
China Open. In team competition, he won a bronze medal with Japanese team at the
Sudirman Cup.
2026 In March, Watanabe competed at the
All England Open, where he upset third seed
Anders Antonsen in the opening round before being eliminated in the quarter-finals by Canada's
Victor Lai. He was also a member of the Japanese squad that secured the nation's first men's team title at the
Asia Team Championships. In April, Watanabe competed at the
Asian Championships in
Ningbo, China, defeating
Zaki Ubaidillah in the first round before falling to
Loh Kean Yew in the second round. ==Awards and nominations==