In February 2020, Nakashima received a wildcard into the Delray Beach Open (his first ATP main draw event), where he reached the quarterfinals,[6] defeating Jiří Veselý and Cameron Norrie before falling to Yoshihito Nishioka.

[7] Later in the year, on his Grand Slam main draw debut as a wildcard at the US Open, Nakashima defeated Paolo Lorenzi[8] before being beaten by fifth seed and eventual runner-up, Alexander Zverev.

Nakashima qualified for the 2021 Next Generation ATP Finals as the fourth seed in recognition of his breakout success in the year among players aged 21 and under.

[20] In his group, he notched wins against Juan Manuel Cerúndolo[21] and Holger Rune,[22] taking him to the semifinals, before he lost to eventual finalist Sebastian Korda in five sets.

[24] At the 2022 French Open, Nakashima reached the third round of a major for the first time, where he lost to 3rd seed Alexander Zverev.

[25] At the 2022 Wimbledon Championships, he reached the fourth round of a Major for the first time in his career defeating Daniel Elahi Galán.

Seeded fifth at his home tournament, the 2022 San Diego Open, he reached his third ATP final after defeating Christopher O'Connell.

[37] At the ATP 500 2024 Barcelona Open Banc Sabadell, he defeated second seed Andrey Rublev, for his second top 10 win in his career.

[41] He reached the second round at the 2024 French Open with a win over wildcard Nicolas Moreno de Alboran, [42] before losing to eighth seed Hubert Hurkacz.

[43] At the 2024 BOSS Open in Stuttgart, he reached the semifinals defeating seventh seed Jan-Lennard Struff by walkover and returned to the top 65 in the rankings on June 17, 2024.

[44] At the next grass tournament the following week, the 2024 Queen's Club Championships, he defeated wildcard Dan Evans by retirement, using special exempt to enter the main draw.

At the 2024 Hangzhou Open he reached his 14th ATP Tour quarterfinal, saving two match points against qualifier Coleman Wong.

[51] At the next Asian swing tournament, the 2024 Japan Open Tennis Championships, he continued his good form, defeating seventh seed Frances Tiafoe in straight sets in the first round.

[54] Nakashima excelled in singles play for the Smash and also paired up with Rajeev Ram throughout the season in men's doubles to help Chicago earn a No.

The Smash defeated the Orlando Storm to earn a spot in the final, but ultimately fell to the New York Empire in a Supertiebreaker.