Last season's runner-up Marin Cilic suffered a surprise second-round defeat in the BMW Open after going down in straight sets to his fellow Croatian Ivan Dodig.
Cilic, who also reached the final in Munich in 2010, was beaten 6-4 6-2 in 78 minutes by world number 56 Dodig, who will now face fifth seed Alexandr Dolgopolov in the quarter-finals.
Ukrainian Dolgopolov was pushed all the way in his second-round match with Russian Dmitry Tursunov before winning through 7-6 (7/2) 7-6 (7/3).
Defending champion and home favourite Philipp Kohlschreiber, who also won the clay-court tournament in 2007, had it much easier in booking his place in the last eight as he swept aside Kazakh qualifier Evgeny Korolev 6-2 6-4 in 63 minutes.
Next up for the German fourth seed is Serbia's Viktor Troicki, a 6-4 6-4 winner over Czech Radek Stepanek.