Del Potro eased by sixth-seeded Nicolas Almagro 7-6 (9-7), 7-6 (7-1) while Raonic was a 7-6 (7-4), 6-1 winner over Ivan Dodig.
Del Potro had 12 aces and faced just one break point on the day. He won 75 percent of the points on serve, losing 19 points over 12 games serving, but he only solved Almagro's serve once as Almagro won 70 percent of the points on serve.
That left each set to be decided in tiebreakers. Only in the second-set tiebreaker was either player able to gain much of an advantage.
Raonic had 12 aces and won three-quarters of the points on serve Saturday. After taking the first set in a tiebreaker, Raonic was able to finish off the only two breaks of the match and take the second.
Sunday's championship match will be the second career meeting between del Potro and Raonic. Raonic won a tight 7-5, 6-4 match in Montreal this year.
Del Potro and Raonic have reached four finals in 2013. Del Potro won at the Netherlands and in Washington in moving to 15-6 overall in ATP title matches.
Raonic won at San Jose, Calif., and last week in Bangkok. He is 5-4 in ATP finals.
They both lost finals to Rafael Nadal this year.
Sunday's Japan Open titlist will take home $312,000 while the runner-up earns $140,700. Almagro and Dodig each were given checks for $66,650 as semifinalists.
Puzzle-maker slips 'Murdoch Is Evil' into Rupert Murdoch's Sunday Telegraph
Police: Sword-wielding man demanded free tacos