Three-time Grand Slam champion Stan Wawrinka lost in the first round of the Monte Carlo Masters as he made his return to Tour-level tennis after 13 months out.
Kazakhstan's world number 36 Alexander Bublik beat the Swiss 3-6 7-5 6-2.
Wawrinka, 37, suffered a foot injury in early 2021 and had two operations.
He returned to competition at a Challenger Tour tournament in Marbella in March, where he was also eliminated in the first round.
"I think the body's getting much better," said Wawrinka. "I'm still far away from where I want to be, but I think I'm [going in] the right direction.