Radwanska overpowered Angelique Kerber 6-1, 6-1 Friday, while Petrova defeated Samantha Stosur 6-4, 6-2 in the other semifinal. Stosur took an injury timeout last in the first set to address a pulled abdominal muscle.
Petrova will play for her 12th career title. It's the second time she's beaten two top-10 players in the same week -- No. 7 Sara Errani and No. 9 Stosur -- but she's never beaten three.
"Nadia has been playing great tennis the whole week," said third-ranked Radwanska. "I'm very, very happy I could reach the final and fight for the title again. I'll try my best."
Radwanska faced little resistance against the fifth-seeded Kerber, who advanced to the semis when top-ranked Victoria Azarenka withdrew on Thursday after a bout of dizziness in her third-round match on Wednesday.
Radwanska broke Kerber six times, including in the final game when she converted her fourth match points after tough resistance from the German. Radwanska said the match was tougher than the score suggested.
"We played before and it's always tough against her," Radwanska said. "I had to run a lot. She's a great player and I really had to stay focused."
Radwanska is trying to become the first player here to win back-to-back titles since Lindsay Davenport in 2003-04.
Petrova set the pace early when she broke Stosur in the first game and had nine aces en route to the victory in Ariake Coliseum. Any chance for a comeback by Stosur was dashed when Petrova broke the ninth-seeded Australian in the seventh game of the second set to go up 5-2.
"When I arrived here, I wasn't sure if I was going to be able to play at all because I didn't feel well," Petrova said. "But I got stronger and stronger with every match and am happy to be in the final."
Stosur, who upset second-seeded Maria Sharapova in the quarterfinals on Thursday, suffered an abdominal injury early in the match Friday.
"I did something to myself in the third game of the match," Stosur said. "I kept thinking it would go away, but it didn't seem to. I got some treatment for it and was able to keep playing, but it's not the ideal situation."