The 54th-ranked Chung made it two wins out of two in the round robin format being used in the inaugural edition of the under-21 version of the ATP Tour finals.
Chung beat Russia's Andrey Rublev 4-0, 4-1, 4-3 (7/1) in their Group A match a day after rallying from a set down to get past Canadian Denis Shapovalov.
"The score looks easy, but it was not easy on the court. Every point was a fight," said 21-year-old Chung who is guaranteed of finishing top of the group.
Coric was up two sets to one and 3-2, 40/15 and serving for the match, but thanks to the no-advantage scoring system, when he reached deuce, everything was on the line.
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One more point and he could force a tie-break, and from there a fifth set, but Medvedev could not save a third match point.
"I think it was really physical," Coric said. "The first two sets were extremely tough."
Jared Donaldson is out of the running after a second loss, falling to Khachanov 4-1, 4-3 (7/3), 4-2.
Pre-tournament favourite Rublev, ranked 37th and winner of his first title in Umag this year, had won his opener against Italian wildcard Gianluigi Quinzi.
In the second Group A match, Shapovalov beat Quinzi 4-1, 4-1, 3-4 (5/7), 4-3 (7/5).
Rublev can still make the semi-finals on Thursday when he plays Shapovalov but former Wimbledon junior champion Quinzi is out of the running.
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