[13] In January 2010, he had surgery on the shoulder injury that has plagued him for years, and won the regional round of the National Sports Festival all by RSC in the lightweight division.

He beat Isaac Dogboe 10–9, then was initially declared the loser in his second fight by Magomed Abdulhamidov of Azerbaijan due to controversial judging and refereeing.

Shimizu defeated Algeria's Mohamed Ouadahi 17–15 in the quarter-finals[16] but lost to Great Britain's Luke Campbell 20–11 in the semi-finals which won him the bronze medal.

[21] Shimizu made his professional debut against In Kyoo Lee on September 4, 2016, on the undercard of the WBO super-flyweight title bout between Naoya Inoue and Petchbarngborn Kokietgym.

[22] He won the fight by a fifth-round knockout, flooring Lee with a left hook to the body, which prompted his corner-men to throw in the towel.

[23] Shimizu faced the 6–3 Carlo Demecillo in his second professional appearance, which took place on the undercard of the December 30, 2016, WBO super-flyweight title bout between Naoya Inoue and Kohei Kono.

[30] Shimizu made his second title defense against Kyung Min Kwon on March 26, 2018, in his return to the Korakuen Hall in Tokyo, Japan, where he first won the belt.

[36] Shimizu was booked to challenge the reigning WBO Asia Pacific super-featherweight champion Joe Noynay on July 12, 2019, on the Rob Brant vs Ryota Murata II undercard, at the Edion Arena in Osaka, Japan.

[38] Shimizu made his fifth OPBF featherweight title defense against Kyohei Tonomoto on July 16, 2020, after a year-long absence from the sport.

[40] Shimizu was booked to face the WBO Asia Pacific featherweight champion Musashi Mori in his sixth title defense on May 21, 2021.