By Martyn Herman
MILAN (Reuters) - South Korea's Chung Hyeon topped Group A in the Next Gen ATP Finals with his third straight victory on Thursday and was joined in the last four by Russia's Andrey Rublev who ended the hopes of Canadian teenager Denis Shapovalov.
Chung was pushed hard by Italian qualifier Gianluigi Quinzi before winning 1-4 4-1 4-2 3-4(6) 4-3(3) in the first match to go past two hours at the inaugural event showcasing the world's best players aged 21 and under plus a batch of new rules.
Rublev, at 37th the highest-ranked player in the tournament, looked in trouble against Canadian teenager Denis Shapovalov in the evening session when he needed treatment on blisters to his feet after taking the third set.
He lost the fourth set in a flash but the rugged 20-year-old rallied to win a deciding set tiebreak, winning 4-1 3-4(8) 4-3(2) 0-4 4-3(3) to take runners-up spot in Group A in another match that went past two hours.
"You have to be focussed 100 percent from the beginning because even if you're winning, suddenly in one, two minutes you can start to lose easily," Rublev said.
Another Russian, world number 65 Daniil Medvedev beat already-eliminated American Jared Donaldson 3-4(3) 4-2 4-3(1) 4-0 for his second victory in Group B.
He had an anxious wait to find out if that was good enough for a place in the last four, but Croatia's Borna Coric did him a huge favour by coming back from two sets down to beat Karen Khachanov 3-4(3) 2-4 4-2 4-0 4-2 for his third win.
Chung will face Medvedev in Friday's semi-finals after Coric takes on Rublev.
Chung offered a new twist to the debate about shot-clock, one of several innovations being tested including short sets played to four games and no advantage points.
The 21-year old is a big fan of the scoreboard ticking down for the 25-second rule between points -- saying it meant he knew exactly how long he had to clean his distinctive white-framed spectacles.
In the past he has incurred the wrath of the chair umpire by going over time, but avoided that fate on Thursday in a match that lasted two hours and six minutes.
"I like shot clock, because sometimes I get a warning because I have to clean my glasses," said Chung, who has worn prescription sports glasses throughout his career and only took up tennis to improve his eyesight.
"I get the warnings all the time in long matches. So I like the shot clock."
All eight group matches before Chung's afternoon clash with Quinzi had been completed in less than two hours - averaging one hour 26 minutes as the ATP experiments with speeding up play.
(Reporting by Martyn Herman, editing by Ed Osmond and Ken Ferris)