German wildcard Michael Berrer ousted 2009 champion Jeremy Chardy of France 6-2 6-7 (7-1) 6-3 to reach the Mercedes Cup quarter-finals on Wednesday.
The 153rd-ranked Berrer saved all four break points he faced against the third-seeded Chardy.
Berrer will next face Roberto Bautista Agut of Spain, who rallied to beat the eighth-seeded Martin Klizan of Slovakia 6-7 (7-3) 6-1 6-4.
Second-seeded Philipp Kohlschreiber beat qualifier and German compatriot Nils Langer 7-5 6-2.
Another German, sixth-seeded Florian Mayer, was eliminated by Gael Monfils of France 6-3 6-0. Monfils will next play Kohlschreiber.
Kohlschreiber had won just four matches in his seven previous visits to Stuttgart.