Sharapova will face seventh-seeded Jelena Jankovic in Saturday's final at Ariake Colosseum, hoping to break a title drought that extends back to April last year. She has won four of her previous five matches against Serbia's Jankovic, who defeated China's Li Na 6-4, 6-3 in Friday's other semifinal.
"She's a very solid player," Sharapova said of Jankovic. "She's a great mover and makes you hit a lot of balls. The points are never easy so it will be a tough match."
Radwanska saved two match points in the final game to pull back to 30-40 but Sharapova closed out the match with a crosscourt backhand.
Sharapova has struggled since returning from a 10-month injury layoff but has a record of strong performances in Japan. She won her first tournament as a professional at the Japan Open in 2003 and won this tournament in 2005, defeating Lindsay Davenport in the final.
Sharapova lost to American Melanie Oudin in the third round of the U.S. Open last month and said that gave her time to get ready for this event.
"I've done way better than I thought," Sharapova said. "When you lose early in a Grand Slam, you have more time to prepare but you never know how that will translate in a match."
Jankovic took a medical timeout early in the second set but worked through pain in her win over Li.
Li had just broken Jankovic to go up 2-1 when the Serbian player took a timeout to have her right arm treated. Jankovic broke back in the fourth game and never looked back.
"I started to feel some pain in my arm," the former top-ranked Jankovic said. "The ball was heavy in this humidity. I received some treatment and tried not think about it and just fight for every point."
Jankovic has won two titles this year at Cincinnati and Marbella.
"If I can win my third title of the year, that would be pretty good," Jankovic said. "It's been a difficult year so it would be nice to finish strong."
Jankovic, the highest seeded player left in the tournament, said she was looking forward to facing Sharapova.
"It will be a difficult match," Jankovic said. "She is a strong hitter. I haven't played her in a while and don't know how she is playing so it will be an interesting match."
Top-ranked Dinara Safina of Russia, Venus Williams and French Open champion Svetlana Kuznetsova all lost their opening matches after first-round byes.