Two batters later, Hunter Ditsworth belted an opposite-field shot to give Southwest a 2-0 margin, but Japan would answer with plenty of its own power.
"I was concerned a little bit after the second homer, but I maintained my composure and started to hit my corners," Tomii said. "I felt fine."
A three-run rally with two outs in the second inning, keyed by a two-run triple from Keitaro Miyahara, put Japan ahead, 3-2, and that would hold up.
Japan's bats fully erupted in the fourth inning, as it scored four runs on three long homers -- by Daisuke Hashimoto, Natsuki Yajima and Miyahara -- that landed halfway up the hill at Lamade Stadium, effectively putting the game away.
"I was in pain, but going through tough practices in Tokyo, I learned to block out the pain," said Miyahara, who homered after fouling a ball hard off his leg. He finished a single short of a cycle, going 3-for-4 with five RBIs.
Tomii continued Japan's run of dominant pitching, which was even more outstanding than the team's bats. Tomii gave up just two runs on three hits while striking out nine in the complete-game effort.