Dominic Thiem shocked No. 1 seed Roger Federer 3-6, 7-6(7), 6-4 Saturday in the Mercedes Cup semifinals in Stuttgart, Germany.
Thiem will play Philipp Kohlschreiber in Sunday's final.
Federer controlled the first set, sailing after taking a 2-1 lead by breaking Thiem's serve.
However, Thiem rolled out to a 5-0 advantage in the second set before holding on to win a tie-break. Federer had a chance to close out the match with a 6-5 lead but Thiem forced the tie-break and later hit a winner to take the set. Rain then suspended the match.
After play resumed, Thiem took a 4-3 lead in the third before the weather delayed the match again. He managed to hold serve twice more to finish off Federer and advance to the final.
"I can't believe it," the No. 3-seeded Thiem said. "To beat Federer on grass is a dream. You need a little bit of luck to beat him on his best surface. I'm the happiest person on the Earth now."
Thiem is 2-1 head-to-head against Federer, all this year.
In the other semifinal, Kohlschreiber beat Juan Martin del Potro 6-3, 6-4 in a rain-delayed match.
Kohlschreiber broke del Potro's serve twice in the first set to take a 5-3 advantage before closing it out.
In the second set, rain stopped the match for three hours at 2-2. After they returned, Kohlschreiber clinched it by breaking del Potro on his third match point.
Muller had 22 aces, including four straight while trailing 5-6 in the second set. He also won 96 percent of his first-serve points (49 of 51).
Karlovic was also strong with his serve with 21 aces and winning 42 of 47 first-serve points.
"I'm happy I'm in a final," Muller said. "It was a very tough match."
On the women's side, No. 3 seed Kristina Mladenovic beat her best friend, Belinda Bencic, 2-6, 6-3, 6-4 to advance to her second career final.
Mladenovic will take on Coco Vandeweghe for the championship.
Bencic dominated the first set in just 33 minutes, but Mladenovic charged back to take the next two.
"It was definitely a very tough match, and a very tricky one," Mladenovic said. "As probably everybody knows, Belinda is my best friend on the tour and in real life. It's a lifetime relationship. It's not easy."