Canada's Denis Shapovalov dropped a 1-4, 4-3 (5), 4-3 (4), 4-1 decision to South Korea's Hyeon Chung in his opening match Tuesday at the inaugural Next Gen ATP Finals.
Shapovalov, 18, is the youngest player in a field that includes eight of the best 21-and-under players on the ATP Tour.
The tournament continues through Saturday at the Fiera Milano stadium.
Shapovalov, from Richmond Hill, Ont., reached a career-high ranking of No. 49 last month. He currently holds the No. 51 position.