Even a week ago, the US Open winner from 2020 had been eliminated in the first round at the sandplace tournament in Belgrade, his first ATP tour appearance since June 2021.
Thiem had long had to pause because of a complicated hand injury.
At the end of March, the 28-year-old had completed his first official match after the long break at the Challenger tournament in Marbella, but also had to take a defeat.
After that, he was thrown back by a coronavirus infection.