Red Sox prospect Triston Casas hit a go-ahead, two-run homer in the fourth inning, Nick Allen also went deep and the United States rallied past defending champion South Korea 4-2 Saturday night to finish the group stage of the Olympic baseball tournament with a 2-0 record.Nick Martinez (1-0), who left the major leagues for Japan after the 2017 season, struck out nine in five innings. Scott McGough, Edwin Jackson, Anthony Gose and David Robertson finished a five-hitter for the Americans, who struck out 14.The United States, which beat Israel 8-1 in its opener, earned Sunday off as the Group B winner and plays Group A winner Japan (2-0) on Monday night in the start of a double-elimination second stage. South Korea (1-1) faces the Dominican Republic (1-1) on Sunday.Martinez, a right-hander who turns 31 next Thursday, was 17-30 for Texas from 2014-17 and signed with the Pacific League’s Hokkaido Nippon-Ham Fighters after the Rangers demoted him to Triple-A 13 times. He switched to the Fukuoka SoftBank Hawks this season and is 7-2 with a 2.03 ERA.While Major League Baseball refused to allow 40-man roster players at the Olympics and many teams blocked top prospects from participating, Nippon Professional Baseball stopped its season.Martinez allowed one run and four hits. South Korea, which started seven left-handed hitters, scored in the first after Park Hae-min led off with a three-hopper to shortstop and beat Allen’s throw for an infield single.Park took third on a single by Lee Jungh-hoo, a son of former Korean League MVP Lee Jong-beom, and scored when Kim Hyun-soo, who played for Baltimore and Philadelphia in 2016-17, bounced into a forceout.McGough, a 31-year-old right-hander, had his only big league experience in six games with Miami in 2015. He is in his third season with the Pacific League’s Yakult Swallows. Gose reached 98.9 mph in his 1 1/3 innings; no other pitcher has topped 94.5 mph.Robertson got the save after allowing a walk, double and Oh Jaeil's sacrifice fly.Side-arming South Korean right-hander Ko Youngpyo (0-1) gave up three runs and four hits in 4 2/3 innings, struck out six and walked none.Ko allowed one baserunner in the first three innings, then plunked Eddy Alvarez on the left thigh with a breaking ball leading off the fourth.Casas, a 21-year-old who was the 26th overall pick by Boston in the 2018 amateur draft, reached down for a 2-2 breaking ball on the inside corner at the knees and drove it about four rows into the right-field seats, Casas is in his first season at Double-A Portland.Allen, a career minor leaguer playing independent ball in Midland, Texas, jumped on a hanging breaking ball in the fifth, driving it 10 rows into the left-field seats.Alvarez’s single chased Ko, and singles by Alvarez and Tyler Austin against Go Woosuk made it 4-1.JAPAN 7, MEXICO 4Former Central League MVP Tetsuto Yamada had four RBIs and broke open the game with a three-run homer, and Hayato Sakamoto went deep off former big league pitcher Manny Bañuelos.Masato Morishita (1-0), a 23-year-old right-hander who is reigning Central League Rookie of the Year, allowed two runs and five hits in five innings. Ryoji Kuribayashi got three straight outs for the save.Joey Meneses, a 29-year-old in Double-A with Boston and the 2018 International League MVP, had three RBIs for Mexico, including a two-run homer in the eighth off Kaima Taira.Mexico faces an elimination game on Sunday against Israel (both 0-2).Mexico starter Juan Oramas (0-1) allowed two runs and four hits in three innings.Morishita, who pauses his windup with his left leg bent at a 90-degree angle and only then finishes his delivery, gave up an RBI single in the first to Meneses.Takuya Kai, the 2019 Japan Series MVP, hit a tying single in the second, and Hideto Asamura chopped a run-scoring comebacker in the third for a 2-1 lead.Yamada, the 2015 Central League MVP and a six-time All-Star, turned on a fastball at the letters in the fourth from Arizona minor leaguer Édgar Arredondo for a 5-1 lead.Mexico had runners at the corners with no outs in the bottom half. Adrián Gonzalez, a five-time All-Star whose last big league season was 2018, brought in a run when he grounded into his second double play to go along with two strikeouts.Samamoto homered in the seventh off Bañuelos, who had a 6.31 ERA in a big league career than ended in 2019. Yamada added an RBI single in the eighth.Oliver Pérez, a 40-year-old left-hander released by Cleveland in May, pitched got two outs in the ninth and did his familiar leap over the foul line.