Top-seeded Alex de Minaur of Australia beat Alejandro Davidovich Fokina on Tuesday in their opening match at the Next Gen Finals in Milan.
De Minaur won eight of the last nine games against the Spaniard to prevail 4-2, 3-4 (5), 4-1, 4-1 at the round-robin event, which features a unique five-set format where each set is first to four games.
The 20-year-old De Minaur was runner-up last season at the year-end event for the best under-21 players.
Miomir Kecmanovic of Serbia won the other Group A match, beating Norwegian Casper Ruud 4-3 (5), 4-3 (5), 4-2.
In Group B, 18-year-old Italian Jannik Sinner delighted his home crowd by upsetting second-seeded Frances Tiafoe of the United States 3-4 (4), 4-2, 4-2, 4-2.
It was the first win for an Italian at the event, which is in its third year.
Also, Mikael Ymer of Sweden beat Frenchman Ugo Humbert 4-3 (2), 1-4, 4-2, 4-1.