The 87th-ranked Haas defeated defending champion Philipp Kohlschreiber 7-6 (5), 7-5 on Saturday to face second-seeded Federer, who earlier eased past Mikhail Youzhny of Russia by 6-1, 6-4 to reach his seventh final.
"To be able to play Federer again is simply top," said Haas, the oldest player in the singles draw at age 34.
Kohlschreiber had knocked out Rafael Nadal on Friday, when 2009 winner Haas defeated No. 3 seed Tomas Berdych.
After winning the first set on a tiebreaker, Haas got the decisive break in the 11th game of the second set to lead 6-5 before wrapping up the match.
"I said it all week, that Tommy's playing fantastic tennis," Kohlschreiber said.
Earlier, the second-seeded Federer broke Youzhny twice to wrap up the first set in just 21 minutes. The Swiss was in such control that one fan implored him to slow down.
The 16-time Gram Slam champion responded by breaking twice more, before dropping serve to lead 5-3.
Federer wasted three break points in the next game, but sealed the match in the following one with his eighth ace.
"It was nice to come out and play some decent tennis after yesterday's shootout drill," Federer said, referring to his tiebreaker win over Milos Raonic of Canada in the quarterfinals.
The 29-year-old Youzhny has never beaten Federer in 13 attempts.
"Mikhail played a great tournament, and he deserves respect for the way he fought back in the second set," Federer said.
Federer's only final defeat at Halle was to Lleyton Hewitt in 2010. He is bidding for his fifth title of the season after wins in Rotterdam, Dubai, Indian Wells and Madrid.
