Top seed Stefanos Tsitsipas impressed in his opening match of the Next Gen Finals round-robin stage as he recorded a four-set victory over Jaume Munar.
Tsitsipas won the first ATP Tour title of his career in Stockholm last month and his confidence has remained high, the Greek beating seventh seed Munar 4-3 (7-5) 4-3 (7-3) 3-4 (4-7) 4-2 in Milan.
There was little between the pair in an entertaining Group A encounter, but when it mattered Tsitsipas had more quality, his backhand working smoothly to convert openings.
Munar saved three match points in the fourth set but he was unable to stop Tsitsipas completing the victory.
It is Frances Tiafoe who tops Group A after the opening round, though, the American very much in form in his 4-1 4-2 2-4 4-3 (12-10) win over Hubert Hurkacz.
Tiafoe's groundstrokes were a class above those of his Polish opponent and he raced into a two-set lead in just half an hour.
Hurkacz forced a fourth set and a tie-break but Tiafoe's power proved too much, a huge ace followed up by a rasping forehand as he secured the win.
Last year's beaten finalist Andrey Rublev came from 2-1 down to open his tournament with a win over Taylor Fritz.
Rublev rued a string of missed opportunities during their one hour and 55 minutes on court, the fifth seed only converting three of his 16 break chances.
Alex de Minaur also started Group B with a victory, the Australian having no problems as he beat wildcard Liam Caruana 4-1 4-1 4-2.
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