Continuing as a juvenile in 2009–2010, he won the bronze medal at the Upper Great Lakes Regionals before finishing sixth at the 2010 U.S. Championships.

In 2011–2012, Hiwatashi moved up to the intermediate level, winning the gold medal at the Upper Great Lakes Regionals and the 2012 U.S. Championships.

He advanced to the novice level in 2012–13, winning the gold medal at the Upper Great Lakes Regionals, the Midwestern Sectionals, and the 2013 U.S. Championships.

Coached by Alexandre Fadeev in Wilmette, Illinois,[5] Hiwatashi was scheduled to make his ISU Junior Grand Prix (JGP) debut in Mexico in early September 2013 but sustained a medial malleolus fracture in his left foot during an official practice at the competition.

Later that month, he was selected to replace the injured Nathan Chen at the 2016 World Junior Championships in Debrecen, Hungary.

[6] In March at the World Junior Championships, he placed sixth in the short program and third in the free skate to win the bronze medal behind Daniel Samohin of Israel and Nicolas Nadeau of Canada.

He competed at the 2016 CS Warsaw Cup, placing ninth, and finished fifteenth at the 2017 U.S. Championships on the senior level.

Concluding the fall season at the Junior Grand Prix Final, he placed sixth overall after struggling in both programs.

[11] In his final event of the season, Hiwatashi competed at the 2019 World Junior Championships alongside countrymen Alexei Krasnozhon and Camden Pulkinen.

He placed second in the short program, briefly holding the junior world record until it was reclaimed minutes later by Pulkinen.

However, his strong placement in the short program combined with his performance in the free skate allowed him to claim victory overall, and he became the World Junior Champion, ahead of Savosin and bronze medalist Daniel Grassl of Italy.

"[25] Scheduled to compete at the 2022 U.S. Championships in hopes of qualifying for the American Olympic team, Hiwatashi was forced to withdraw due to a positive COVID test.

[28] Following the season, he moved to Kyoto, Japan to train under Mie Hamada, Hiroaki Sato, and Satsuki Muramoto at the Kinoshita Academy.

[28] At the 2023 CS Golden Spin of Zagreb, Hiwatashi placed eight in the short program after falling on his opening quad toe loop.