Milan - South Korea's Hyeon Chung upset top-seeded Russian Andrey Rublev to claim his first title at the Next Gen ATP finals in Milan on Saturday.
The bespectacled 21-year-old rallied from a set down to see off Rublev, the world number 37 who reached the US Open quarter-finals this year, 3-4 (5/7), 4-3 (7/2), 4-2, 4-2 in just under two hours.
Chung, ranked 54, sealed victory on his third match point after Rublev saved two to came back to 3-2 in the final set.
The former student of the Nick Bollettieri Tennis Academy in Florida - who turned to a mental coach to help him deal with the pressure of tennis - receives the top prize of $390 000 but no world ranking points for winning the Under-21 tournament.
It was Chung's third victory over Rublev, whom he beat earlier this week and also saw off in Winston-Salem on the ATP Tour in August.
Russia's Daniil Medvedev won the third-place playoff after Croatia's Borna Coric pulled out injured.
The ATP used the inaugural tournament to test a number of innovations with the promising young players of the season competing in the round robin format with the first to four games in each set winning - a tie-break is used when the scores reach 3-all.
Changes also included no line judges with all calls made using Hawk-Eye Live, no lets, sudden-death points at deuce and a shot clock to ensure a 25-second rule between points to keep things ticking along.
Results from the final day of the Next Gen ATP finals Under-21 tournament in Milan on Saturday:
Final (x denotes seeding)
Hyeon Chung (KOR x6) bt Andrey Rublev (RUS x1) 3-4 (5/7), 4-3 (7/2), 4-2, 4-2