Ichiro Suzuki broke out of his recent slump while Shuichi Murata and Kenji Johjima smashed home runs before the game was ended on the mercy rule after seven innings at Tokyo Dome.
"I think Ichiro was under a lot of stress," Japan coach Tatsunori Hara told reporters. "But his batting tonight set the tone for the team."
The Koreans should still comfortably beat China on Sunday to advance along with Japan while securing a shot at instant revenge over their fierce rivals in Monday's final Pool A game.
Japan's leadoff batter Suzuki ignited a three-run first innings with a line-drive single to deafening roars from a crowd of 45,000.
South Korea responded immediately with a towering two-run shot into the left field seats from Kim Tae-kyun, who pumped his fists and roared in delight as the ball sailed over the fence.
However, Japan let rip in the second with a five-run outburst to break the game open, capped by a three-run homer from third baseman Murata, going deep for the second game running.
Boston Red Sox pitcher Daisuke Matsuzaka, tournament MVP in Japan's 2006 triumph at the inaugural WBC, recovered from a shaky start to keep Korea's big guns in check through four innings.
Ichiro Boost
Suzuki's performance will give Japan a major boost, the Seattle Mariners outfielder ending his hitless streak, including warm-up games, at 16 at-bats as he went 3-for-5.
"We weren't worried about Ichiro not hitting," said Matsuzaka. "But when he starts hitting the team catches fire."
Catcher Johjima's two-run homer in the sixth gave Japan an 11-run advantage over last year's Beijing Olympic gold medalists before the game was called with Korea never threatening to rally.
"We're Olympic champions but we know Japan are a dangerous team," said South Korea coach Kim In-sik. "They have got more of their Major League players here and that makes a difference."
Japan had beaten China 4-0 in the curtain raiser for the 16-team tournament on Thursday, while the Koreans crushed Taiwan 9-0 in their opening game.
China eliminated Taiwan with an upset 4-1 win earlier on Saturday. Pool C and D games were beginning in Canada and Puerto Rico later on Saturday.
Pool B play in Mexico gets under way on Sunday while the second round onwards will be held in the United States.
