Up next for the 37-year-old, who is looking to win a 102 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 semifinal.
Federer is preparing for the Wimbeldon tournament as he attempts to win the English Grand Slam tournament for the eighth time.
"Every time I won in Halle I went on to play really well at Wimbledon".More news: Uruguay cruise past Ecuador in Copa America
Federer endured a rough ride in the early rounds in Halle, battling to hard-fought, three-set victories over Jo-Wilfried Tsonga and Roberto Bautista Agut.
The Swiss maestro said his game was where he wanted it to be after overcoming Pierre-Hugues Herbert at the semi-final stage, but Goffin forced the only break points of the first set.
"I was able to run away with the lead getting the break early on in the second and I think that maybe calmed down my nerves", Federer said.
In the second set Federer raised his game a couple notches and immediately got the rewards, as he broke the Goffin serve, after the Liege born man double faulted.More news: Fan hospitalized after being hit by foul ball at Dodger Stadium
"As soon as Roger is ahead with a break, he started to serve better, be a little more aggressive and it was tough to come back".
World No. 33 Goffin added: "I thought I started really well, there was great intensity in the first set".
Federer first played the Halle Open as an 18-year-old in 2000, when both his current coach, Ivan Ljubicic, and Goffin's coach, Thomas Johansson, played it, too. As less than ten days are left for the grass-court major to begin, the eight-time champion on the SW19 grass, Federer flies to London with sanguinity and with a second seed badge.
The now 37-year-old Lopez was playing his first final since defeating Marin Cilic in the decider two years ago and was made to work hard by Simon, himself no newbie at 34.More news: Trump signs new sanctions against Iran
Ashleigh Barty became only the second Australian woman to top the world rankings when she won the Birmingham WTA title on Sunday, beating Julia Goerges 6-3, 7-5.