Ji came off the bench to turn Melbourne City defender Rebekah Stott inside out before firing a shot past New Zealand goalkeeper Erin Nayler in the 74th minute.
Another substitute, Moon Mi-ra, pounced on an error from Nayler to score South Korea’s second goal with three minutes remaining in regulation.
Australia, which won its opening games against South Korea and New Zealand, was playing Argentina in a later match. The Matildas need only a draw to clinch the tournament.
The Cup of Nations is a warm-up tournament for the World Cup in France in June.
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