Juan Martin Del Potro made a triumphant return to clay with a 7-6 (7-2) 6-4 first-round win over Dustin Brown at the BMW Open, but second seed Gael Monfils was forced to withdraw through injury.
Del Potro is appearing in just his fourth competition since returning from almost a year on the sidelines with a wrist problem in February, with the Munich event his first on clay since the Rome Masters in 2013.
Brown saved four break points to send the opening set to a tie-break, but the Argentine quickly moved into a 4-0 lead and never looked back.
World number 337 Del Potro almost let a 5-1 lead in the second set slip, but won match point at the fourth attempt to complete the win.
He will face Jan-Lennard Struff in the next round after the German came from behind to upset seventh seed Vasek Pospisil 6-7 (5-7) 6-3 6-2.
Igor Sijsling secured a comfortable 6-2 6-4 victory over Marco Cecchinato, but will not meet Monfils next after the Frenchman was forced to surrender his place due to a groin problem sustained while practicing. Lucky loser Jozef Kovalik takes his place.
Fifth seed Fabio Fognini beat wildcard Maximilian Marterer 6-3 6-1, while Victor Estrella set up a second-round meeting with top seed David Goffin by downing Cedrik-Marcel Stebe 7-5 6-2.
In second-round matches, third seed Dominic Thiem defeated Santiago Giraldo 7-5 6-4 to set up a quarter-final meeting with Ivan Dodig, who saw off Thomaz Bellucci 7-6 (7-5) 6-3.
Alexander Zverev fought from behind to advance at the expense of Malek Jaziri 4-6 6-3 6-2.