Ivan Dodig of Croatia ousted second-seeded countryman Marin Cilic 6-4, 6-2 in the second round of the BMW Open on Wednesday.
Dodig reached his third quarterfinals of the year at the expense of Cilic, who reached the final in 2010 and 2012.
Dodig, ranked 56th in the world, also defeated the Russian two-time champion Nikolay Davydenko in the first round.
Defending champion Philipp Kohlschreiber shot 12 aces in beating Kazakhstan qualifier Evgeny Korolev 6-2, 6-4.
The fourth-seeded Kohlschreiber, who also won the title in 2007, next plays Serbia's Viktor Troicki, who defeated Radek Stepanek of the Czech Republic 6-4, 6-4.
Fifth-seeded Alexandr Dolgopolov of Ukraine also progressed, but was pushed hard by Dmitry Tursunov of Russia before prevailing 7-6 (2), 7-6 (3).