Sasaki was dominant, clocking 21 of his 66 total pitches at or above 100mph according to the radar gun shown on the TV broadcast.
Japan moved to 3-0 in Pool B and is assured a spot in the quarter finals.
Pitching on the 12th anniversary of the 2011 earthquake and tsunami that claimed the life of his father and grandparents, Sasaki, 21, allowed two hits and one run.
Marek Chlup doubled and scored in the first inning on a fielding error to give the Czech Republic (1-1) the 1-0 lead.
Japan's offense, kept off-balance by Ondrej Satoria's diet of off-speed pitches and a fastball that never hit 80 mph, finally broke through with three runs in the third inning.
Shohei Ohtani's one-out RBI double helped charge a four-run fourth. Ohtani also stole third base.
Satoria had four strikeouts and left with a memorable souvenir. He fanned Ohtani, who spun himself to the ground and lost his helmet.
Japan can complete a perfect 4-0 run through pool play by beating Australia on Sunday.
Chinese Taipei 9, Netherlands 5
Yu Chang launched a second-inning grand slam and Chinese Taipei handed the Netherlands its first loss in Pool A action in Taichung City.
Chieh-Hsien Chen doubled, tripled and drove in a run and Li Lin went three-for-five with two RBIs for Chinest Taipei (2-1), who never trailed after scoring five runs in the second inning.
Jurickson Profar singled and scored on a Didi Gregorius single to give the Netherlands (2-1) a 1-0 lead in the first inning. Ray-Patrick Didder hit a solo homer in the ninth inning.
Puerto Rico 9, Nicaragua 1
Marcus Stroman gave up one run over four-and-two-thirds innings, while MJ Melendez, Emmanuel Rivera and Christian Vazquez each drove in a pair of runs as Puerto Rico won the opener in Pool D play in Miami.
Nicholas Padilla picked up the win after recording the final out of the fifth inning. Puerto Rico broke open a tie game with a five-run fifth inning as Melendez delivered a two-run single to centre.
Elian Miranda hit a home run for Nicaragua, while JC Ramirez (0-1) was charged with four runs on three hits and a walk in a third of an inning after he opened the fifth on the mound.
