German world No 3 Alexander Zverev came from a set down to beat Tallon Griekspoor on Friday, making the last four in Munich for the first time in seven years.
Zverev, the first seed, won 6-7 (6/8), 7-6 (7/3), 6-4 after fighting back from the brink of defeat in the second set to stay on track to win the title he claimed in 2017 and 2018.
Griekspoor, who knocked Zverev out of Indian Wells last month, served for the match at 5-4 in the second set but delivered a sloppy game to let his opponent off the hook.
"At the end of the day, I'm very happy to have won the match. He played very well tactically and made it very difficult for me," Zverev told Sky Germany, saying he was "totally mentally exhausted".
"Today I had to somehow get through. Hopefully, I'll play better tennis tomorrow."
Zverev will face Hungarian Fabian Marozsan, who beat Belgium's Zizou Bergs 6-3, 7-6 (7/4) in Friday's last quarterfinal.
If he wins, Zverev will bid to win the tournament for a third time on his 28th birthday on Sunday.
Joining Zverev in the semis is world number 15 Ben Shelton, who beat Italian Luciano Darderi 6-4, 6-3.
Shelton advances to face Argentina's Francisco Cerundolo, ranked 22nd, in the last four.
It is the second time the 22-year-old has reached a semifinal this year, after his run to the last four at the Australian Open in January, where he lost in straight sets to eventual winner Jannik Sinner.
After the win, Shelton found his father Bryan in the crowd and said into the microphone: "We're back, big dog".
"I feel pretty good moving around, sliding, getting into drop shots, defending, playing offensively. I was kind of like a deer on ice a couple of years ago, but I've evolved for sure," he added.
"I'm not the player I want to be yet but I'm working towards it for sure."
Cerundolo also came through in straight sets on the Munich clay, winning 6-2, 6-4 against David Goffin.