South Korean Hyeon Chung became the first player to book his ticket to the semi-finals of the Next Gen ATP Finals in Milan on Wednesday.
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.
Croatia's Borna Coric beat Daniil Medvedev 4-3 (7/5), 2- 4, 4-1, 4-2 but will have to wait until Thursday to book his berth after a fightback by the Russian who saved two match points.
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.
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."
Coric will need to get two sets Thursday against Karen Khachanov to seal one of the two Group B spots in the semi- finals on the final day of round-robin play.
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.
The tournament has brought together eight emerging stars tipped to succeed the likes of Rafael Nadal, Andy Murray, Novak Djokovic and Roger Federer in the future.
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