2008 Summer Olympics

He competed at the 2008 Summer Olympics at featherweight (– 57 kg) but lost his first bout 9–12 to Yakup Kılıç.^[4] He was not happy about the judgement,^[10][11] and turned down the offers to sign professional contracts in order to win a medal at the Olympics.^[9] He secured the bronze medal in the featherweight division in the 2009 Asian Amateur Boxing Championships in Zhuhai, China,^[12] and won the National Sports Festival of Japan in the lightweight division to become a three-division champion of that title in the same year.^[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.^[14] Only one week after that, he moved down in weight class to the bantamweight division because the featherweight division has been removed in the AIBA events for the 2012 Summer Olympics since September 2010,^[14][15] and participated in the Asian Games, but lost in the quarterfinals.^[12]

2012 Summer Olympics

Shimizu qualified for the 2012 Summer Olympics in the bantamweight 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. At the beginning of the third round, he was trailing Abdulhamidov by 7 points; however, in the last 2 minutes of that round, Abdulhamidov began to tire. Shimizu repeatedly knocked his opponent down, 6 times in total.^{[Video 1]} AIBA officially overturned the ruling after an appeal by the Japanese Coach stating that Abdulhamidov should have been given at least 3 warnings and therefore disqualified.

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. It was Japan's first men's boxing medal since 1968.^[5] Shimizu made a significant contribution to Japan's 2012 Olympic boxing team as an icebreaker.^[17] However, the medal was lost in spring 2013.^[6]

He decided to join APB (AIBA Pro Boxing) for the right to fight for the gold at the 2016 Summer Olympics.^[18] So, he retired from the Japan Ground Self-Defense Force, and entered Miki House in April 2014.^[6]

Early career

Shimizu decided to turn professional after failing to qualify for the 2016 Summer Olympics.^[19] He began training at the Ohashi Boxing Gym,^[20] and acquired his professional boxing license on July 29, 2016.^[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. Prior to the knockout, Lee was knocked down in both the second and fourth rounds.^[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.^[24] He won the fight by a third-round knockout, stopping Demecillo with a right hook to the jaw.^[25] Shimizu was next scheduled to face Takuya Yamamoto on May 21, 2017.^[26] He won the fight by a first-round technical knockout.^[27]

OPBF featherweight champion

Shimizu was booked to challenge the reigning OPBF featherweight champion Sa Myung Noh on October 2, 2017. The title fight was scheduled as the main event of the "61st Phoenix Battle", which took place at the Korakuen Hall in Tokyo, Japan.^[28] He captured the belt with a fifth-round knockout of Noh. Shimizu pressured Noh from the outset of the fight and knocked him down it the fourth. Although he was unable to finish him in that round, he managed to do so at the 1:54 minute mark of the very next round.^[29] Shimizu made his first title defense against Eduardo Mancito on December 30, 2017, on the undercard of the Naoya Inoue and Yoan Boyeaux WBO super-flyweight title bout. He knocked Mancito down with a right hook in the first round and dropped him twice more in the seventh round, before finally finishing the Filipino challenger with a right hook at the 2:08 minute mark of the seventh round.^[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.^[31] He first knocked Kwon down with a left straight in the sixth round, before forcing the referee to wave the fight off with a flurry of punches in the eight round.^[32] Shimizu made his third title defense against the 16–3–1 Shingo Kawamura on August 17, 2018.^[33] He won the fight by a fourth-round knockout. Shimizu first staggered Kawamura with a left straight, before knockoing him out with a strike to the body.^[34] Shimizu made his third title defense against the undefeated Takuya Uehara on December 3, 2018, in the main event of "66th Phoenix Battle".^[35] He won the fight by a third-round technical knockout.^[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.^[37] Despite coming into the fight as a favorite, Shimizu suffered an upset stoppage loss. Shimizu was knocked down twice in the first round and failed to recover in the remaining five rounds, as he lost the fight by a sixth-round technical knockout.^[38]

Shimizu made his fifth OPBF featherweight title defense against Kyohei Tonomoto on July 16, 2020, after a year-long absence from the sport.^[39] He won the fight by a seventh-round technical knockout.^[40] Shimizu was booked to face the WBO Asia Pacific featherweight champion Musashi Mori in his sixth title defense on May 21, 2021. He won the fight by unanimous decision. Two of the judges scored the fight 118–110 in his favor, while the third judge scored the fight 116–112 for Shimizu.^[41] Shimizu vacated the WBO Asia Pacific title on November 26, 2021, six months after he had won it.^[42]