Ruud confidently swatted aside an error-prone Rublev 6-2 6-4 on Saturday to set up a clash against Novak Djokovic.
He has had a great year - reaching his first Masters 1000 final in the Miami Open, before making it to the finals of the French Open and US Open - and will be looking to cap it off with victory at the season-ending championship.
"I think I surprised myself for sure," Ruud told reporters.
"It was a tough start of the year for me because I twisted my ankle in Australia the day before my match in the Australian Open, so I had to pull out. I guess when I was on the plane back home, I did not think I would be sitting here in this position.
"I think when the draw was made here, I guess I was the least favourite to reach the final, compared to all the other great players who I consider are maybe better hard court indoor players than myself."
Djokovic is in the running to win a record-equalling sixth ATP Finals title on Sunday, and Ruud, who is 0-3 in head-to-head battles against the Serb, heaped praise on his opponent ahead of their meeting.
"Novak, in a final on an indoor hard court here, is one of the toughest players you can play in the history of our sport," Ruud said.
"I know that I will need to come up with something I have not done yet against him because I haven't been able to beat him. Just try to believe that I can win. It's not going to be easy, obviously, but I will give it a shot, my best shot."
