The World No 1 apologised to the crowd, explaining he had felt unwell since the previous day, with his condition worsening on the day of the match.
Sinner's early retirement raises concerns about his fitness for the upcoming US Open, particularly his singles campaign, though he confirmed it was illness rather than injury.
His participation in the new-look US Open mixed doubles tournament, where he was scheduled to partner Katerina Siniakova, is now uncertain.