[6] At the age of 12, he passed the selection test for the Masaaki Morita Tennis Fund and went to the United States to train at the IMG Academy in Florida.

[11] In February, Mochizuki made his ATP main draw debut as a wildcard at the 2021 Singapore Tennis Open where he lost to Altug Celikbilek in straight sets.

[12] He won his first-ever trophy as a professional by winning the Open Città della Disfida Challenger in Barletta, Italy, defeating the Argentine Santiago Rodriguez Taverna in straight sets.

In July, he made his Grand Slam debut after qualifying for the main draw of the 2023 Wimbledon Championships but lost in the first round to 16th seed Tommy Paul in straight sets.

After nine attempts, he finally won his first match at the ATP Tour level, beating Tomás Martín Etcheverry in straight sets.

[21] Following a quarterfinal showing at the Sydney Challenger he reached the top 130 in the rankings and 13th in the 2023 Next Generation ATP Finals race.

[22] He made his debut in the main draw at the 2024 Australian Open as a lucky loser but lost to Tomáš Macháč in straight sets.