Roberto Bautista Agut, David Goffin and Philipp Kohlschreiber, the three seeded players in action on Wednesday, all advanced to the quarter-finals of the ATP clay-court tournament in Munich.
Third-seeded Bautista Agut breezed past qualifier Radek Stepanek 6-0 6-3 and No.5 Kohlschreiber dispatched wild card entry Alexander Zverev 6-2 6-4.
Kohlschreiber is a two-time champion, while the 18-year-old Zverev is the youngest of eight teenagers in the Top 200 of the ATP Rankings.
Fourth-seeded Goffin needed three sets to get past Simone Bolelli 7-6 (7-5) 6-7 (7-4) 6-1 and will next face Kohlschreiber.
Victor Estrella Burgos overcame Janko Tipsarevic 6-4 6-7 (7-1) 7-5 to set up a match against Bautista Agut.
aces to beat Souza.
Top-seeded Andy Murray will be in action on Thursday against German qualifier Mischa Zverev.