Federer is through to the Halle final for the seventh time after a 6-1, 6-4 victory against unseeded Russian Mikhail Youzhny in the first semi-final.
The Swiss second seed 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 ten 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.
The Swiss world number three will now meet Haas, who beat defending champion Philipp Kohlschreiber in straight sets in the second semi-final.
Kohlschreiber had beaten Rafael Nadal in the quarter-finals but was edged out 7-6 (7/5), 7-5 by Haas, a former world number two who is appearing in the tournament as a wildcard.
Haas also has previous here, having beaten Djokovic in the final in 2009.
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