Dominic Thiem beat Roger Federer for the second time in a month, seeing off the Swiss top seed 3-6 7-6 (9-7) 6-4 in a rain-interrupted Mercedes Cup semi-final in Stuttgart.
Federer was unable to end the contest in straight sets, missing on a pair of match points in the second-set tiebreaker.
"I was incredibly nervous. I was lucky in the tiebreaker but you need good luck to beat the best player on grass," Thiem said.
The Swiss was playing his first event since Rome in May on clay after dealing with back pain which forced him to miss the French Open.
The contest was interrupted twice for a total of an hour by the passing rain showers.
Thiem will aim for his fourth trophy of 2016 on Sunday when he takes seventh-seeded Philipp Kohlschreiber, who had to wait for three-and-a-quarter hours due to rain to finish off a 6-3 6-4 defeat of wild card Juan Del Potro.
"This was one of the best feelings I've had in tennis. For sure this was one of my best matches - I didn't know I could play so well on grass," Thiem said after enduring two rain delays and a fightback form Federer from 5-1 down in the second set which almost needed the match in the top seed's favour.
Federer had only himself to blame for failing to fight through for the win after he blew a pair of match points in the second-set tiebreaker.
"It was a good match in tough condition," Federer said. "We were playing in the rain at the end of the second set, we had rain breaks after the second and another in the third.
"Coming back in the second set was good but it was tough to lose in the breaker.
"I can play better, but he came up with some good shots when he needed them."
Thiem has now beaten an injured Federer on clay in Rome last month as well as on the icon's preferred grass.
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