The nine-time champion has extended his Swiss Indoors win streak to 23 matches as he defeated Tsitsipas, 6-4 on two sets, to reach the final for the 15th time, piling up his 50th win if 2019.
The 38-year old will play for his 10th title at his home tournament when he faces Alex de Minaur.
Swiss-born Federer has now made the Basel final in his last 13 appearances and 15th overall.
His last downfall came in the 2013 finals after he was defeated by Juan Martin de Potro.
Federer will have a new match up in the finals with De Minaur, who has won trophies in Sydney, Atlanta, and Zhuhai.
“He’s one of the fastest players,” Federer said. “I’m very excited to be in the final again. I was not sure how it would go this week, so I’m really happy,” he added.
De Minaur is the first Australian to reach the Basel final since Mark Philippoussis finished runner-up in 1997.
The 28th-ranked De Minaur asked when he won the semi-final match up against Opleka, “Nothing can prepare you for that serve,” de Minaur said.
“Reilly’s an incredibly tough competitor who never gives up. I’m just happy to be in the final. I guessed right a few times and got lucky. I was just lucky to have gone the right direction, I was just hoping not be hit by any of those serves.”