Watanuki first main draw match victory on the ATP Tour came at the 2018 Rakuten Japan Open over Robin Haase as a qualifier.

[4] Watanuki entered the 2021 Winston-Salem Open as a lucky loser and won his first match against Jaume Munar.

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.

[8] 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.

[9] In January, Watanuki qualified for the 2023 Australian Open to make his Grand Slam debut.

[10][11] He beat Arthur Rinderknech in the first round in straight sets for his first Major win before losing to Sebastian Korda.

117, he also qualified for his second Masters at the 2023 Mutua Madrid Open and defeated Corentin Moutet in the first round before losing to world No.

[13] At the 2023 BOSS Open he qualified and won his first round match against wildcard Feliciano López.

336 and using a protected ranking and qualified for the main draw after beating Otto Virtanen and Hong Seong-chan.