Trailing by a goal at the start of the last quarter of their preliminary league match against the team that had lost all the matches, the Koreans went all out into the attack and were rewarded by two goals, with the winner coming from skipper Jung.
With one match to go, South Korea now have seven points from four outings and are assured of a spot in the semifinals of the six-nation event.
The semifinal lineup will only be decided after the last bunch of round-robin games tomorrow.
Japan finished their matches with five successive losses, despite a fine show in the last two outings that they lost by one goal each to defending champions Pakistan and South Korea. Kim Juhun gave South Korea the initial lead in the fourth minute with a field goal, only to see Japan equalise through Kazuma Murata's penalty corner conversion four minutes later.
The Koreans went ahead again in the 32nd minute when captain Jung scored the first of his two open-play goals.
Japan shot back to score twice in in successive minutes to then go ahead as captain Hiroki Sakamoto (44th minute) and Kenta Tanama (45th minute) made flash strikes to give them a 3-2 lead at the end of the third quarter.
Pushed back, the Koreans bounced back to draw 3-3 parity within a minute of resumption of play as Kim Seung-Hoon scored with a snap shot.
Captain Jung fired his team's match-winner in the 49th minute in open play. The Koreans then denied Japan another equaliser with some resolute defence.
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