Sinner was unstoppable as he dispatched Ruud 6-1 6-2 in Saturday's evening session at a packed Inalpi Arena and will be favoured to become Italy's first winner of the prestigious year-ender.
The second semi-final was a stark contrast to the first in which Fritz and world No.2 Alexander Zverev were involved in a ferocious slug fest, the American winning 6-3 3-6 7-6 (7-3).
Taylor Fritz celebrates his win over Alexander Zverev. (AP PHOTO)
Sunday's title match will be a rematch of September's US Open final, in which Sinner beat Fritz in straight sets.
Fritz is the first American to reach the final of the tour finale since James Blake in 2006 and will hope to become the first American to lift the trophy since Pete Sampras in 1999.
"I have believed that I belong, that I'm one of the best players," Fritz said. "It's not results-based. It's more I can feel how I'm playing. This week is huge."
Sinner's stupendous year may have been clouded by an anti-doping controversy but on the court he is finishing the season like an express train and has won 25 of his last 26 matches as he heads toward's January's Australian Open title defence.
The 23-year-old was razor-sharp against Ruud and was in complete control after breaking serve in the second game. He has reached the final without dropping a set.
"I raised my level and I was intense with each shot," said Sinner, who beat Fritz in the group stage in Turin.
Sinner lost in the final to Novak Djokovic last year having beaten the Serb in an earlier group match.
"It's a similar circumstance to last year. I'll just try to play my best tomorrow and hopefully it'll be a good day," Sinner said.
"I've grown a lot as a player."
Zverev, who won the Paris Masters in the build-up to Turin, looked like the biggest threat to Sinner's title hopes.
The 27-year-old German, twice a winner of the year-ender, had not reckoned on an inspired Fritz however.
Fritz had won his previous three matches against Zverev and played what he described as a "perfect first set" to move ahead.
Zverev hit back though to set up a deciding set and had enough chances to have taken the win.
Fritz got ahead in the tiebreak and admirably held his nerve, clinching the win with a thumping forehand winner.
"I found myself in some tough spots in the third, both of us really started to get on each other's serve even more, said Fritz, who will end the year at a career-high world No.4.
"I told myself just to give him nothing from the back."
"I've been playing the top guys at the big events a lot lately," Fritz said. "I'm getting more comfortable and confident in those moments."
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