Two favourites have bowed out on opening day at tennis' Next Gen finals with Russia's Karen Khashan and Canadian Denis Shapovalov both losing opening group matches in Milan.
The event limited to the top eight ATP players aged under 21 is serving as a grand experiment for a new set of rules designed to speed up the game.
Changes include six instead of 12-game sets, no lets and strict enforcement of the 25-second period between the end of the previous point and the start of the subsequent one.
Players are also able to speak with their coaches in the stands on headphones at the end of sets.
Fellow Russian Daniil Medvedev scored a defeat against Khachanov 2-4 4-3 (8-6) 4-3 (2-7) 4-2 at the Fiera Milano.
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Korea's Chung Hyeon beat Shapovalov 1-4 4-3 (7-5) 4-3 (7-4) 4-1.
Khachanov has 25 wins this season, the most of anyone in the eight-man field. His hard-court record stands at 6-19 in 2017.
Medvedev has played six quarter-finals, a semi-final and a final last January in Chennai.
Shapovalov, who made his entrance onto the big stage by beating Rafael Nadal at his home Masters 1000 in Montreal, was unable to earn a victory against his Asian opponent.
Victory for Chung took 95 minutes, with the winner going without even one ace while Shapovalov fired a dozen.
Serb Borna Coric won his opening match over American Jared Donaldson 4-3 (7-2) 4-1 4-3 (7-5).
