"I played well in the tiebreak and at the important moments," del Potro said. "In this kind of match, one or two points can be the difference so I was just trying to stay focused until the end."
Del Potro is seeking his third title of the season following wins in Rotterdam and Washington.
Raonic, who is coming off a win at the Thailand Open last week, won six straight points in the tiebreaker and cruised through the second set to reach the Japan Open final for the second straight year.
The 22-year-old Canadian, who is 11th in the world rankings, broke Dodig twice in the second set to win the last five games, finishing the match with three straight aces.
"He sort of let me get back into it in the tiebreak," said Raonic, who had 12 aces. "After I got that first set, I was getting closer and closer to breaking his serve. He gave me freedom and I started swinging more and more freely after that."
Raonic lost in the final of last year's tournament to local favorite Kei Nishikori, who fell to Almagro in the quarterfinals.
Del Potro said he was looking forward to facing the big-serving Canadian.
"Milos is playing very well, he already won in Thailand," del Potro said. "I know his game, he is serving well and it will be tough to break his serve. I will have to play even better than today and I know he will be a difficult opponent for me."