Starting his racing career at age 5 when his father introduced him to racing karts, Hiroki Otsu graduated from the Suzuka Circuit Racing School Formula (SRS-F) in 2013. The following year with the support of Honda, he joined JAF F4, and in 2015 finished third overall in FIA-F4. From 2016, Otsu moved up to the Japanese Formula 3 championship, finishing fifth in 2017. In 2018 he raced in the SUPER GT GT300 class with Modulo drago CORSE, ranked 14th with a third-place podium in his debut year, and 17th place in 2019. Moving up to the GT500 class in 2020, Otsu is now racing with Modulo Nakajima Racing with teammate Takuya Izawa, completing the year 12th overall with a pole position and podium finishes.

From 2021, Otsu will begin racing full-time in SUPER FORMULA, in addition to SUPER GT.

In Round 6, he achieved a pole-to-win finish, and in 2022 SUPER FORMULA, he climbed onto the podium in 2nd place at the final race at Suzuka.

In 2023, his SUPER GT team changed to ARTA and he marked his first win in Round 5. He was active in the title race right up to the final round. In 2024, he continues to compete in SUPER GT for ARTA. He competes in Super Formula from Round 4 for TGM Grand Prix.