In his early days at the Eta Jima Academy, he earned the nickname "gamecock" because of his stamina and vehemence[clarification needed] for life.

After graduating from the 67th Naval Academy Class (ranking 152 out of 248 Cadets) and being appointed an ensign (少尉) in 1939, Sasai enrolled in the 35th Aviation Student Course.

On December 10, 1941, Ensign Sasai, as a member of the Tainan Air Group, which would become one of the Navy's premier fighter units, participated in the Pacific War's opening actions in the Philippines during the attack on Luzon, but he was forced to abort the mission due to engine trouble.

Sixteen days later, Sasai shot down a Curtiss P-40E of 17th Squadron with the machine guns of his Mitsubishi A6M Zero.

[2] The Tainan Kōkūtai (Air Group) was transferred to Lae on Papua New Guinea in March 1942, reorganizing with new officers.

Among his Tainan equals, Sasai was known as "Flying Tiger", a reference to the tiger-head belt buckle he wore always, a gift from his father.

Sasai was now flying with experienced pilots in his squadron, including Saburō Sakai, Enji Kakimoto and Toshio Ohta.

He was just following procedure, but we knew he was secretly proud of us.On April 11, Lieutenant Sasai led nine Zeros scouting over Moresby in three "V" formations.

Sasai led 12 Zeros providing fighter escort for 21 bombers from Rabaul heading for Port Moresby.

According to Sakai, Sasai's decision was poor, but he and the rest of the pilots sympathized with him because bombers had been hitting their base regularly, and they all wanted to get a piece of the B-17s that had been giving them so much trouble.

During this same engagement, Saburō Sakai, leading the second section in Sasai's squadron, sustained grievous injuries from the return fire of carrier-based bombers he had mistaken for Wildcats.

"[7] On August 26, 1942, on his 76th sortie with the Tainan Group, Sasai led eight Zeros escorting Betty bombers to Henderson Field, a main focus of months of fighting in the Guadalcanal campaign, as it enabled U.S. land-based airpower to hinder the Japanese attempts at resupplying its soldiers.

The Zero turned to engage from overhead; the Grumman pulled its nose into the vertical, firing its six .50-caliber machine guns.

The lack of self-sealing fuel tanks made Japanese Zeros easy to ignite with a few rounds of incendiary ammunition.

Sasai's Zero exploded in a ball of fire, showering parts onto the beach of Guadalcanal in front of hundreds of U.S. Marines.

He was probably the target of Captain (later Major General) Marion E. Carl, an 18.5-victory ace of Marine squadron VMF-223, one of the few Midway-based pilots to survive the Battle of Midway.