Five-time champion Roger Federer will face "mystified" German Tommy Haas in the final of the ATP grasscourt tournament after the two tour veterans clinched straight-sets semi-finals wins on Saturday.
Federer reached the Halle final for the seventh time after a 6-1, 6-4 victory against unseeded Russian Mikhail Youzhny.
Wildcard Haas, a former world number two, defeated compatriot Philipp Kohlschreiber, the quarter-final conqueror of Rafael Nadal, 7-6 (7/5), 7-5, and admitted later it was a mystery how he managed to reach the final.
Second-seeded Federer showed few signs of fatigue despite scraping past Canada's Milos Raonic in a third-set tie-break in the last eight on Friday, and needed barely an hour to see off the world number 31 Youzhny.
Federer fired down eight aces, and remains on course to win the Wimbledon warm-up event for the sixth time.
He later claimed that saving three break points in the fifth game of the opening set was vital to his ultimately comfortable win.
"The key for me was coming back from 0-40 then holding that and going 4-1," said the 30-year-old, who was beaten by Novak Djokovic in the semi-finals at the French Open last week.
"That was a big 10 minutes for me and then, once in the lead, I played a bit more aggressive and he was missing quite a few shots, making it easier for me.
"Overall, I think I played a good tough solid match, especially after yesterday's shoot-out (against Raonic)".
Federer's previous victories in Halle came in 2003-06 and again in 2008, while he also lost the final to Lleyton Hewitt in 2010.
Haas, the champion in 2009 when he beat Djokovic in the final, admitted he had been surprised by his run to the final.
"It's unexpected, it's a bit of mystery how I got here. But I am delighted to be playing good tennis at Halle," said Haas.
The 34-year-old, who will be playing his first final since beating Djokovic at the same venue in 2009, has won just two of his 12 matches against Federer.
Two of his defeats came in the Halle semi-finals in 2005 and 2006.
Their last clash was in the Wimbledon semi-finals in 2009, again won by the Swiss.
Haas, now ranked 87 in the world, recently missed 14 months of action after undergoing hip surgery, the latest in a long line of career injury problems for the German.
He qualified for the French Open where he went on to reach the third round.