Russian Petrova upset eighth seed Samantha Stosur 6-4 6-2 in a surprisingly one-sided match to advance to her first final at the WTA premier five event in Tokyo.
Arms, neck and shoulders criss-crossed with taping, third seed Radwanska was in awesome form, the big-hitting Pole completing an easy win in just 59 minutes.
"Even if it was 6-1 6-1 I had to run a lot to win that match," said Radwanska, who earlier this week became the fourth player to qualify for the season-ending WTA Championships in Istanbul, with four spots still up for grabs.
"The games were long so I started to try something different, mixing my game up. It worked. I'm just happy I could play my best tennis."
German Kerber failed to hold serve in the first set, and was left to watch as several Radwanska bombs flew past her racquet as her serves fell short and were smashed back with interest.
Radwanska, mixing up her game with lobs and delicate drop shots, wrapped up victory by forcing the fifth seed into a wild backhand that landed in the tramlines on her fourth match point.
Petrova, the 17th seed, chasing her second title of the season, came into the match with a 5-3 record over Stosur, last year's US Open champion.
Stosur committed a string of unforced errors and Petrova closed out the match following another errant forehand from the Australian after one hour, 47 minutes.
"Sam is a tough opponent," said Petrova. "She has a huge serve, a huge forehand. You have to be alert, you have to play your best tennis and I was able to accomplish that."
Nine of the world's top 10 women made the trip to Tokyo, though the hard-court tournament lost its two top seeds, Victoria Azarenka and Maria Sharapova, on Thursday.