Kazakh Alexander Bublik broke Alexander Zverev once in each set to power past the German 6-3, 7-5 on Saturday and reach the Halle Open final.
The 26-year-old Bublik, who regularly delivered first serves at speeds of more than 220 km per hour, gave his opponent no chance on his own serve to make his first final of the year as the grass season heads towards Wimbledon in just over a week.
Zverev, a two-time finalist in Halle, quickly dropped a break behind with Bublik racing to a 4-1 lead in the first set.
The German's return was no match for the Kazakh's thundering serve and Bublik quickly wrapped up the first set in 35 minutes.
Bublik, who reached his second tour semi-final of the season after Italian Jannik Sinner was forced to retire injured, had two more break chances at 3-3 but Zverev dug deep to stay in the contest.
However, the Kazakh played a flawless game at 5-5 to snatch the break he wanted with a superb crosscourt forehand winner and then served out the match.
The world number 48 will face either Roberto Bautista Agut of Spain or Russian Andrey Rublev in Sunday's final.
Alcaraz sets up Queen's final against De Minaur
Spain's Carlos Alcaraz continued to thrive on London grasscourts in the build-up to Wimbledon when he beat unseeded American Sebastian Korda on Saturday to reach the Queen's Club final.
The 20-year-old lost his opening set at Queen's earlier in the week but has since looked ominously good and proved too powerful for Korda, winning 6-3 6-4.
In his first ATP Tour final on grass, Alcaraz will face Australian Alex de Minaur who earlier beat Danish second seed Holger Rune 6-3 7-6(2).
Should Alcaraz win Sunday's final he would reclaim the world number one ranking from Novak Djokovic and would be seeded one at Wimbledon which begins on July 3.
Korda, 22, was at full stretch to try to keep Alcaraz at bay in the opening set but still dropped serve twice, although he did also manage to break the Spaniard.
Alcaraz began to unleash some heavy groundstrokes and showed plenty of grasscourt acumen despite being a relative novice on the surface. He forged an early break ahead in the second set and never looked in danger from that point on.
"Every match I feel better and I feel like I've been playing on grass for 10 years," Alcaraz, who will attempt to emulate Rafa Nadal by winning Wimbledon next month, said on court.
"I'm playing great and feeling great. Very happy to make my first final on grass and even more so at Queen's.
Fellow 20-year-old Rune had not dropped a set on the way to the semi-finals but found the livewire De Minaur a step too far as he fell short of also reaching his first final on grass.
World number 18 De Minaur, who put out home favourite Andy Murray earlier in the week, is aiming for his second career title on grass having won at Eastbourne in 2021.
He is the first Australian to reach the final at the Queen's pre-Wimbledon event since Lleyton Hewitt in 2006.
Two breaks of serve allowed the seventh seed to cruise through the first set but he was pushed much harder in the second with both players forced to save break points.
However, De Minaur's greater experience on the slick lawns showed as he polished off the tiebreak.
"I play pretty decent on the grass, I love playing on this court," he said. "Today was my best match so far this week. I just hope that I can go one better tomorrow."
