Fresh from winning the ATP tournament in Cologne last weekend, Germany's Alexander Zverev reached the quarter-finals of the second week on the Rhine with a tough, three-set triumph over John Millman of Australia on Wednesday.
Zverev, 23, ranked seventh in the world, needed nearly two hours to post a 6-0, 3-6, 6-3 victory over the 31-year-old Millman.
The German top seed will meet France's Adrian Mannarino in Friday's quarter-finals.
On Thursday, Japan's Yoshihito Nishioka, who saw off Britain's Kyle Edmund on Tuesday, will play Germany's Jan-Lennard Struff for a last-eight place.
Seventh-seed Struff beat Italy's Marco Cecchinato 6-3, 6-1 in the first round.
Cologne's Lanxess Arena is hosting back-to-back ATP events.
Zverev defeated Canada's Felix Auger-Aliassime last weekend in the final of the first event.