Mischa Zverev ended the Stuttgart International run of German compatriot Tommy Haas as the sixth seed posted a 6-4 6-4 defeat of the popular veteran in the quarter-finals.
Haas was making what was likely his final appearance at the Weissenhof club, where he played the final in 1999.
The 39-year-old was fresh from an upset 48 hours earlier of top seed Roger Federer on the grass.
"By and large, it has been a nice week here," world No.302 Haas said. "Actually, I'm on a good path.
"It's going in the right direction. But no sportsman likes to lose."
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Zverev, who will move to a career-best No.29 in the rankings, will face off in the semi-finals against Feliciano Lopez after the Spaniard hammered 19 aces and saved all nine break points he faced to upset third seed Tomas Berdych 6-7 (7-4) 6-3 6-4.
"He's a great grass player with a totally different game as a lefty," Zverev said after reaching his first Stuttgart semi. "The match will be different, but we will see."
Haas could not rev up his serve, leading to speculation that his problem shoulder might be bothering him again.
The crowd favourite was broken once in the opening set and lost his serve for 4-5 in the second as Zverev served it out.
"I knew a few days ago that I would have to play one of two legends," the winner said. "It was an unbelievable match today.
"It was not easy, I was happy to come through in two sets. I've never beaten Tommy on grass, but he gave me a lesson in our other match on the surface (Halle 2009).
"Tommy showed how well he can play, he has a really effective serve which he can place anywhere."
Haas was the oldest player to reach an ATP quarter-final since Jimmy Connors at 42 in 1995.
Against Lopez, Berdych dropped to 6-7 against one of the few Spaniards who is quite content to play on grass.
Lopez, 35, staged a fighting comeback after losing the opening set in a tiebreaker and eventually prevailed to take his season record to 12-12.
French fourth seed Lucas Pouille held off a fightback attempt from German Philipp Kohlschreiber to earn a 6-4 2-6 6-3 trip to the final four. Kohlschreiber finished runner-up in 2013 and a year ago against Dominic Thiem.
