Up next for the 37-year-old, who is looking to win a 102nd tour-level trophy and move closer to Jimmy Connors' record of 109, is Belgian David Goffin who beat Matteo Berrettini 7-6(4) 6-3 in the other semi-final.
"The last couple of days were hard, that's why I am thrilled to be in the final," Federer, who was facing his 337th different opponent, said.
World number 33 Goffin had earlier ended Stuttgart champion Berrettini's eight-match winning streak, withstanding 14 aces and saving all three break points he faced.
"Matteo is a great player, he's serving so well. I had to be focussed and aggressive, not too passive," Goffin said.
"Otherwise he would have had a couple of chances... I played well from the start. That was the key to stay focused from the start. I'm playing well, more aggressive... so, it's a great feeling this week to be in the final."
Goffin's only victory against Federer in eight meetings came at the ATP Finals in London two years ago, where he eventually finished as runner-up.
The Halle Open is a warm-up tournament for Wimbledon which begins on July 1.
(Reporting by Shrivathsa Sridhar in Bengaluru, editing by Pritha Sarkar)
