Dominic Thiem suffered a 6-3, 3-6, 6-4 loss to John Millman in the opening round of the Serbia Open on Tuesday in his first tour-level match in 10 months.
The 2020 US Open champion and former World No 3 had been sidelined with a wrist injury suffered in a grass court tournament in Marbella in June last year.
The 28-year-old Austrian returned to action in the same Spanish city last month in a second-tier Challenger clay court event where he lost in the first round. He tested positive for Covid-19 the day after that defeat.
Thiem, a two-time French Open runner-up and 2020 Australian Open finalist, has slumped to 54 in the world during his prolonged absence.