The sophomore guard scored a season-high 23 points, including 5 of 6 from 3-point range, against South Dakota on November 27.
Stellar pitching from starter Masato Morishita and relievers keep the Americans off the scoreboard
Ryoji Kuribayashi holds the U.S. in the 10th to pick up the win.
Japan will face the U.S. on Monday for a chance to reach the semifinals.
That Yamamoto started the first of two tuneup games is a fairly strong indication that Inaba plans to have him start the opening game on Wednesday.
Orix Buffaloes ace Yoshinobu Yamamoto has emerged as one of the best pitchers for Samurai Japan.