Top seed Alexander Zverev edged out South African Lloyd Harris 6-4, 3-6, 6-0 to reach the Cologne ATP semi-finals on Friday.
Harris struggled with a leg injury in the final set of the match, something that gave world No.5 Zverev an advantage.
“I have got to do a better job of keeping my focus until the end of a match. I relaxed a little bit and then he picked up his game,” Zverev said after the match.
Zverev will face Alejandro Davidovich Fokina in his third semi-final of the year.
The Spaniard beat Dennis Novak of Austria 6-3, 2-6, 6-3.
Canada’s Felix Auger-Aliassime defeated Radu Albot 6-3, 6-0 to reach the last-four and will face Roberto Bautista Agut, who downed Poland’s Hubert Hurkacz 7-6 (7/4), 5-7, 6-0.