Australian third seed Bernard Tomic and German veteran Tommy Haas, the eighth seed, were both knocked out of the Stuttgart ATP claycourt tournament in the second round on Thursday.
Tomic went down 7-6 (8-6) 6-3 to Brazilian claycourt specialist Thomaz Bellucci while Slovak qualifier Pavol Cervenak, the world No.206, saw off Haas 6-4 6-4.
Bellucci, the world No.64, saved eight of the 10 break points he faced to claim victory in one hour and 47 minutes.
"It was very tough," said Bellucci.
"At the beginning of the match I was a little bit nervous. When I was 1-4 down, I started to play better and in the second set I was feeling more comfortable and more confident."
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.