Taylor Fritz dashed Alexander Zverev's hopes of a grass title in front of home German fans after winning 6-7 (4-7), 6-4, 7-5 in the Halle semi-final on Saturday.
The US fifth seed pounced towards the end of third set to reach the final. German Daniel Altmaier plays the US's Frances Tiafoe in the other semi-final in western Germany later.
Recent French Open champion Zverev, gunning for a second Grand Slam title at Wimbledon starting next Monday, has had a good work-out in the warm-up. But the top seed is still without the Halle title despite four consecutive semi-finals and two lost finals in 2016 and 2017.
Zverev won a tight first set despite needing treatment but let Fritz back in to set up a tense decider. At 5-5, Fritz was 40-0 up on Zverev's serve and eventually grabbed the break before serving out the match in scorching conditions.
Fritz has titles on grass from Stuttgart and Eastbourne over the years and is looking for his first triumph this season on Sunday at the western German event.
