Next Generation ATP Finals favourite Alex de Minaur started his 2019 campaign with a victory over Alejandro Davidovich Fokina in Milan.
De Minaur lost to Stefanos Tsitsipas in last year's final and is the highest-ranked player competing this time around.
The Australian needed four sets to overcome Alejandro Davidovich 4-2 3-4 (5-7) 4-1 4-1 in Group A.
De Minaur, ranked 18th and a three-time winner on the ATP Tour this year, won eight of the final nine games to wrap up the match in style.
"I just had to get in my head that I played a good second set," said De Minaur. "He just came up with big shots in big moments.
"I had to move on and remember what I was doing well the whole match. I felt a lot more comfortable as soon as I got the break [in the third set]."
In the same pool, Miomir Kecmanovic claimed a straight-sets victory over Casper Ruud.
There was only one break of serve in the whole contest, which the Serbian won 4-3 (7-5) 4-3 (7-5) 4-2.
In Group B, Swede Mikael Ymer sealed a 4-3 (7-2) 1-4 4-2 4-1 triumph over Ugo Humbert, and Jannik Sinner closed Tuesday's play by coming from a set down to beat Frances Tiafoe 3-4 (4-7) 4-2 4-2 4-2.