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.
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“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.