The Norwegian, who was the runner-up at both the French and US Opens this season, saw off Nicolas Jarry (26) of Chile 6-2, 3-6, 6-3 in Seoul.
Ruud reached the semi-finals of the season-ending event in Turin last year before losing to Daniil Medvedev.
"I just got the news that I qualified for the ATP Finals in Turin again this year and I'm so much looking forward to coming back to Italy," Ruud told atptour.com.
"I had such a great time last year, it's a great country for tennis."
Ruud is playing in his first singles tournament since the US Open where he had a chance to take the world number one spot.
But he lost to Carlos Alcaraz in the final, with the Spanish teenager instead becoming the youngest top-ranked men's player in history.
Alcaraz and Rafael Nadal have already booked their places in the eight-man field for the ATP Finals which take place from November 13-20.
"My goal for this year is to finish in the top three in the ranking," Ruud added.
"But there are many good players that probably feel the same, so if I can do that I'll be very happy."
The 23-year-old will face Yoshihito Nishioka of Japan for a semi-final spot in Seoul on Friday after overcoming a second-set blip against the 111th-ranked Jarry.
"The first set went very well, but in the second set it was tough to break him. I had to work hard and I was very happy to get the win in the end," Ruud said.
Second seed Cameron Norrie also advanced to the quarter-finals on Thursday by beating Japan's Kaichi Uchida 6-2, 6-2, while third seed Taylor Fritz withdrew due to illness.