Australian third seed Bernard Tomic and German veteran Tommy Haas, the eighth seed, were both knocked out of the Stuttgart claycourt tournament at the second round stage on Thursday.
Tomic went down to a 7-6 (8/6), 6-3 defeat to Brazilian claycourt specialist Thomaz Bellucci while Slovak qualifier Pavol Cervenak, the world 206, saw off Haas 6-4, 6-4.
Bellucci, the world 64, saved eight of the 10 break points he faced to claim victory in one hour and 47 minutes.
In the quarter-finals Cervenak will tackle Argentine second seed Juan Monaco, who breezed past Germany's Tobias Kamke 6-2, 6-3.
"It's been a bad day for me," said Haas. "I wasn't striking the ball well and missed a lot of crucial points."
Last month, Haas defeated Roger Federer to win the Halle grasscourt tournament.