Thiem was playing his first tour-level match since injuring his right wrist last June.
John Millman tainted Dominic Thiem's return from injury by beating the Austrian 6-3, 3-6, 6-4 in the first round of the Serbia Open on Tuesday.
Thiem was playing his first tour-level match since injuring his right wrist last June, and the former world No. 3 showed some signs of encouragement before Millman prevailed in just over two and a half hours.
The 80th-ranked Australian broke in the 10th game of the third set to reach the second round, where he will play Serbia's Miomir Kecmanovic. It was the 32-year-old Millman's first win in four matches against Thiem.
Also on Tuesday, Czech player Jiri Lehecka defeated Henri Laaksonen 6-2, 7-5 to seal a second-round meeting with the second-seeded Andrey Rublev.
Serbia's Laslo Djere defeated compatriot Hamad Medjedovic 6-4, 7-5, Roman Safiullin beat Mikael Ymer 7-5, 6-0, and Brazilian qualifier Thiago Monteiro ousted Portugal's Joao Sousa 6-4, 6-3.
