French Open champion Alexander Zverev will face American Taylor Fritz in the semifinals of the ATP event in Halle after edging Raphael Collignon in straight sets on Friday.
The world No 3 and top seed won 7-6 (12/10), 7-6 (7/3) on the lightning-fast grass, with both men holding their serve throughout the match in the traditional Wimbledon warm-up ATP event.
Zverev converted his second match point against his Belgian rival to win in just over two hours in warm conditions in Germany's west.
"It was an incredible match, with a very high level of play from both players," Zverev said, adding: "those two sets could have easily gone the other way."
The German will face Fritz in the semifinals in his bid to win a maiden grass court title of his career, just weeks after breaking through for a debut Grand Slam victory.
Zverev, who made the final in Halle in 2016 and 2017, has a poor recent record against Fritz, having lost his past six matches against the American.
Earlier on Friday, Fritz battled past Ben Shelton in three sets, exacting revenge on his countryman five days after losing the title match in Stuttgart.
World No 9 Fritz won 6-7 (5/7), 7-6 (10/8), 7-6 (7/3) in two hours and 45 minutes.
Shelton, the world No 5, beat the 29-year-old Fritz to claim his first career title on grass in the Stuttgart ATP tournament last Sunday.
It was a battle between the two American big servers, neither player was broken throughout the match, with each set win coming through tiebreaks.
Shelton triumphed in the opening set. Fritz, a career grass court specialist, fought back to claim the second, saving a match point to force a deciding set.
Shelton fell to the grass midway through the third set and limped back to his bench but was able to continue.
Fritz grew more confident as the match went on and was in control to win the third and final tiebreak.
Later on Friday, world No 4 Felix Auger Aliassime plays Frances Tiafoe and fourth seed Daniil Medvedev faces Daniel Altmaier in the remaining quarterfinals.