Santiago Giraldo of Colombia upset second-seeded Jurgen Melzer of Austria 6-4, 7-5 on Tuesday to advance to the second round of the Mercedes Cup.
Giraldo beat Melzer for the first time in three attempts. The Colombian reached the final in Santiago in February, also on clay.
In another upset, German wild card Cedrik-Marcel Stebe celebrated his first victory on the ATP tour when he beat sixth-seeded Nikolay Davydenko of Russia 3-6, 6-1, 6-4.
The 20-year-old German who comes from near Stuttgart lost his only other tour-level matches, in the first round at Halle and Wimbledon. Both were on grass.
Davydenko was twice a semi-finalist in the clay-court tournament in Stuttgart.
Third-seeded Mikhail Youzhny of Russia, who captured his first career title in Stuttgart in 2002, eased past Germany's Philipp Petzschner 6-3, 6-3.
Jeremy Chardy of France, who won the tournament in 2009, beat Germany's Tobias Kamke 7-6 (1), 6-1.
Two Spaniards, Juan Carlos Ferrero and Pablo Andujar, also advanced, along with Lukasz Kubot of Poland, Fabio Fognini of Italy and Pavol Cervenak of Slovakia.