YOKOHAMA, Japan — The Olympic gold medal in baseball, a sport absent from the Summer Games for 13 years, will come down to the two biggest baseball countries in the world: Japan and the United States.

In a semifinal at Yokohama Baseball Stadium on Thursday, the United States topped South Korea, 7-2, behind another strong all-around performance from a roster of veteran players and young prospects. Joe Ryan, a right-handed starter, allowed one run over four and one-third innings. Four relievers, led by Anthony Gose, combined to allow just one more run the rest of the way.