Two-time French Open finalist Dominic Thiem lost his first competitive match for nearly a year when he went down in three sets to John Millman in the first round at the Serbia Open.
The veteran Australian won 6-3, 3-6, 6-4 in just over two and a half hours.
Thiem has not played since injuring his right wrist at the Mallorca Open in June 2021.
He was set to return at the Cordoba Open in February but was forced to withdraw with a new injury.
After dropping the opening set, the former world number three dug in to claim the second.
In the decider, the 28-year-old Austrian managed to keep pace with Millman but eventually cracked when attempting to level the match at 5-5.
"I've come back from three surgeries so I know how difficult it can be," said Millman after his victory.
"The circuit is going to be better for having Dominic back on the tour. I'll take this win because when Dominic is 100 percent fit and roaring ... I'm going to struggle against him."
Millman will take on the 22-year-old Serb Miomir Kecmanovic in the second round.
Challenge
"He's had a great run over the past few weeks," said Millman of his next opponent. "We've played in practice together and it didn't go so well for me so I'll have to turn it around in match conditions.
He's a good player and it's exciting to test yourself against this younger grade which is coming through."
World number one Novak Djokovic will relaunch his clay court season on Wednesday at the tournament.
The 34-year-old Serb will play compatriot Laslo Djere for a place in the last eight.
Last week at the Monte Carlo Masters, Djokovic lost in three sets in the second round to the world number 46 Alejandro Davidovich Fokina.
“I didn’t like the way I felt physically in the third set,” Djokovic said after the defeat.
“I just ran out of gas completely. Just couldn’t really stay in the rally with him. I mean, if you can’t stay in the rally, not feeling your legs on the clay, it’s mission impossible.”
