German teenager Alexander Zverev continued his rise to prominence by upsetting Roger Federer 7-6 (7-4) 5-7 6-3 to book a place in the Gerry Weber Open final.
Zverev, showing no nerves in front of his home crowd in Halle, took the opening set on a tie-break, teeing up a set point with an ace and celebrating with a fist-pump after Federer dropped a forehand into the net.
But Federer, a record eight-time champion in the tournament, showed all of his experience to stay in the match despite the 19-year-old putting huge pressure on his serve and made a crucial break at 5-5 in the second set with a well-judged drop shot.
Federer, 15 years Zverev's senior, then held his serve to level the match when the youngster could only direct a return into the net, setting up a mouth-watering deciding set.
Zverev saved break points in each of his first two service games and the teenager then broke Federer to go 4-2 up and take full control of the match.
He showed few signs of nerves in closing out the match, serving to love to book his second singles final, while Federer's loss was his first defeat to a teenager since the 17-time Grand Slam winner was beaten by a young Andy Murray in Cincinnati back in 2006.
"I feel pretty good," said Zverev. "I played very good and served well once again. I think that helped me a little bit and I played well in the important moments. It's unbelievable to get a win against Roger - especially on grass."
That result set the stage for Dominic Thiem to confirm the arrival of the next generation against Florian Mayer, but it was not to be as the 32-year-old won 6-3 6-4 to book a meeting with compatriot Zverev in the final.
Thiem seemed unsettled in the early stages of the match and Mayer took full advantage, although it took four attempts to secure the first break of the match in game four.
The 22-year-old Swiss saved two set points on his own serve but Mayer's know-how came to the fore as he served to love to claim the opener.
Thiem's first grass-court defeat of the season was then confirmed in just over an hour when Mayer converted his fourth match point after a second set that had produced three breaks of serve.