2016: Junior No. 2 On the junior tour, Watanuki has a career high combined ranking of No. 2 achieved on 21 March 2016. Watanuki was the winner of the 2016 Campeonato Internacional Juvenil de Tenis de Porto Alegre, a Grade A event in
Porto Alegre, Brazil.
2018–19: ATP & ATP 500 debuts & first win, Maiden Challenger title He made his ATP and ATP 500 debut at the
2018 Citi Open after qualifying. Watanuki first main draw match victory on the
ATP Tour came at the
2018 Rakuten Japan Open over
Robin Haase as a qualifier. He won his maiden title at the
2019 Kobe Challenger.
2021–22: ATP quarterfinal, Two Challenger titles, Top 150 Watanuki entered the
2021 Winston-Salem Open as a lucky loser and won his first match against
Jaume Munar. He lost to
Márton Fucsovics in the second round. He reached the quarterfinals of an ATP tournament for the first time in his career, winning his first two matches in a row at this level, as a lucky loser at the
2022 ATP Lyon Open. First he defeated eight seed
Pedro Martínez for his second top-50 win. Next he defeated
Kwon Soon-woo to set up a quarterfinal with
Alex de Minaur. As a result, he returned to the top 225 in the rankings climbing 40 positions in the rankings at world No. 223 on 23 May 2022. He qualified for his third ATP 500 at the
2022 Citi Open in Washington, D.C., and second time at this tournament, but lost to
Kyle Edmund who was playing his first singles match after his comeback. In November he won two back-to-back titles in Japan at the
Hyōgo Noah Challenger in Kobe and in
Yokkaichi defeating
Frederico Ferreira Silva in both and moved into the top 150 at world No. 145 on 28 November 2022.
2023–25: Major & top 75 debuts, comeback, Masters fourth round In January, Watanuki qualified for the
2023 Australian Open to make his Grand Slam debut. He beat
Arthur Rinderknech in the first round in straight sets for his first Major win before losing to
Sebastian Korda. Watanuki qualified for his first Masters 1000
2023 Miami Open and won his first round match against fellow qualifier
Benoît Paire but lost to 12th seed
Frances Tiafoe. As a result, he moved to a new career high ranking of No. 107 on 3 April 2023. At the
2023 BOSS Open he qualified and won his first round match against wildcard
Feliciano López. He entered the main draw at the
2023 Wimbledon Championships as a lucky loser for the first time at this Major and defeated
Marc-Andrea Hüsler in a five set match for his second Major win. As a result, two weeks later, he reached the top 100 at world No. 99 on 31 July 2023. He finished the 2023 season ranked inside the top 100 at world No. 99. In October, ranked No. 336 Watanuki entered the qualifying rounds of the
Shanghai Masters, using a protected ranking, and qualified for the main draw after beating
Otto Virtanen and
Hong Seong-chan. In the main draw, he upset
Pavel Kotov for his first main draw win in a year, and 32nd seed
Brandon Nakashima to reach the third round Watanuki qualified for the main draw and reached the fourth round of a Masters 1000 for the first time in his career at the
2025 BNP Paribas Open with wins over
Alexander Bublik, his first ATP tour-level win since the Shanghai Masters in 2024, 19th seed
Tomáš Macháč after his retirement, and 16th seed Frances Tiafoe and again moved more than a 125 positions back up to the top 225 at world No. 214 on 17 March 2025. ==Personal life==