With less than a week to go before the start of the US Open, Dimitrov came out sharp, unleashing a torrent of backhanded slices to keep Thiem off balance as the Bulgarian raced through the surprisingly one-sided first set 6-0.
But things took a turn for the worse for Dimitrov at 2-2 in the second and he began to misfire in bunches in an apparent attempt to shorten points.
After a visit from medical staff, he stepped up to the line to serve while trailing 2-4 but decided he was feeling too unwell to continue and shook hands at the net, where he waited for Thiem while hunched over.
The win gives another lifeline to 2020 US Open champion Thiem, who had to save two match points to win his rain-delayed opener against American J.J. Wolf the day before.
Thiem will next face either Italy's Fabio Fognini or Britain's Jack Draper.
Earlier in the day, France's Benjamin Bonzi, Australia's Jason Kubler and Americans Maxime Cressy and Steve Johnson were among those who advanced in straight sets at the ATP 250 tournament in North Carolina.
