Three-time Grand Slam champion Stan Wawrinka fell in the first round in the Sofia Open in Sofia, Bulgaria, losing 3-6, 7-6 (5), 6-4 to Romanian Marius Copil.
The Swiss, who has fallen to 57 in the rankings, won the first set in just 23 minutes after breaking Copil, but the Romanian became more and more comfortable as the match went on and bagged the second set after a tiebreak.
Copil, who reached the tournament final last year, put up stubborn resistance from the baseline in the third set, wrapping up his first career victory over Wawrinka, 33, whose trademark backhand failed to fire.
"It's great to start with a win against Wawrinka," said world No. 56 Copil. "I made a few mistakes at the beginning. I had to change things, I knew I had to stay in the game after losing the first set. ... I served well today. The serve has always been my main weapon."
Elsewhere German qualifier Daniel Brands recorded a big upset against Georgian No. 5 seed Nikoloz Bashilashvili, making the second round with a 7-6 (7), 7-5 win.
Another German qualifier Yannick Maden joined his compatriot with a 6-4, 7-6 (4) win over France's Adrian Mannarino.
Jan-Lennard Struff made it a trio of Germans into the second round with a 6-4, 6-3 defeat of Italian qualifier Stefano Travaglia.
Australia's Matthew Ebden is also through to the second round after a 6-3, 4-6, 7-6 (5) win over Bulgarian qualifier Adrian Andreev.
Seventh seed Gael Monfils progressed in straight sets over Russia's Viktor Troicki while there were also wins for the unseeded Matteo Berrettini and Robin Haase.