Nick Kyrgios described his performance as "pretty average", despite winning on his return after two months out with injury at the Mercedes Cup.
Kyrgios, who said his lay-off had been "brutal", beat German Maximilian Marterer 6-4 4-6 6-3 in the last 16.
"It was a terrible match, the only thing that kept me in it was my serving," the Australian said.
It was the 23-year-old's first match since 13 April due to an elbow injury which ruled him out of the French Open.
"Nothing went very well for me, the good thing is my elbow didn't bother me," Kyrgios said.
"Basically, I didn't feel great out there, today was pretty average. And the last three months have been brutal."
He will play Feliciano Lopez in the quarter-final in Stuttgart after the Spaniard knocked out Frenchman Gilles Simon 7-6 (8-6), 6-4.
Elsewhere, Czech Tomas Berdych beat France's Benoit Paire 7-6 (7-3) 6-4, Canadian Milos Raonic won 6-2 6-4 against Hungarian Marton Fucsovics and Argentina's Guido Pella beat Indian Prajnesh Gunneswaran 7-6 (7-4) 6-3.
Pella will play Roger Federer in the quarter-finals on Friday.