The 35th-ranked Kohlschreiber was seeded sixth at the Stuttgart tournament but Mayer, ranked 21 spots lower in the ATP rankings, beat him 6-4, 6-2.
There was no such upset for fifth seeded Albert Montanes of Spain, who defeated Germany's Philipp Petzschner 2-6, 7-5, 6-2 to advance to the second round.
Story continues below ↓advertisement | your ad hereMontanes' compatriot Daniel Gimeno-Traver eliminated the defending champion Jeremy Chardy of France, 7-5, 6-7 (6), 6-3.
Also into the final 16 were Germans Bjoern Phau and Simon Greul, Spaniards Pablo Andujar and Guillermo Garcia-Lopez, and Argentina's Juan Ignacio Chela and Horacio Zeballos, along with Gilles Simon of France.
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