YOKOHAMA, Japan-Boston Red Sox prospect Triston Casas gave the United States the lead with a two-run homer in the fourth round, and the Americans defeated the South Koreans 4-2 on Saturday to end the group game in the Olympic tournament with the record 2-0.
Nick Martinez (1-0) struck out nine strokes in five innings. Scott McGough, Edwin Jackson, Anthony Gose and David Robertson completed the job, as the American pitchers allowed only five hits.
The United States, which started its tournament by beating Israel 8-1, will be on leave on Sunday when the winner of Group B meets Group A winner Japan at the start of a double elimination round on Monday. South Korea (1-1) meets the Dominican Republic (1-1) on Sunday.
Martinez, who will celebrate his 31st birthday next Thursday, had a 17-30 record with the Texas Rangers from 2014 to 2017. He now plays in Japan, where the professional leagues took a break during the Olympics.
Martinez allowed one run and four hits.
Korean starter Ko Youngpyo (0-1) allowed three runs and four hits in four and two-thirds rounds.
Ko allowed a single runner to hit the tracks in the first three races, then hit Eddy Alvarez in the left thigh to start the fourth.
Casas, who was selected 26th in the 2018 amateur draft by the Red Sox, threw a crazy ball into the stands on the right field. Casas is in its first campaign at the AA level.
Nick Allen hit a homerun to the left field in the fifth inning.
Earlier Saturday, Tetsuto Yamada produced four races, including three in a homeroom, Hayato Sakamoto also threw a long ball and Japan defeated Mexico 7-4 to win Group A.
Masato Morishita (2-0) allowed two runs and five hits in five innings. Beginner Ryoji Kuribayashi struck out three in a row to save the rescue.
Joey Meneses produced three races for Mexico, including two from a homerun against Kaima Taira in the eighth round.
Japan (2-0) erased a three-point delay in the ninth round against the Dominican Republic in their first outing of the tournament.
Mexico meets Israel in an elimination match on Sunday.
Mexico starter Juan Oramas (0-1) allowed two runs and four hits in three innings on the mound.
Morishita gave Meneses a one-round single in the first half to let the Mexicans open the scoring.
Takuya Kai equalized in the second with a single and Hideto Asamura gave Japan a 2-1 lead in the third.
Yamada helped Japan distance itself in the fourth round, when he catapulted a fast ball from Edgar Arredondo over the fence to 5-1.
Mexico had runners at the end of the diamond at the end of the fourth round. Adrián Gonzalez gave a point even though he compromised in a double play.
Samamoto hit a homer in the seventh round against Manny Bañuelos. Yamada added a one-round single at eighth.
Oliver Pérez, a 40-year-old southpaw, knocked out two opponents for Mexico in the ninth round.
Ronald Blum, The Canadian Press