Radwanska, who finished 2-1 in the round robin, will play top-seeded Angelique Kerber in Saturday's semifinals. Kerber won the Australian Open and the U.S. Open this season, and reached the Wimbledon final.
"I think I really have nothing to lose at the moment, especially (since) I'm going to play the best player of the season," Radwanska said of the top-ranked German. "She's played incredible tennis this year."
Earlier, Svetlana Kuznetsova earned first place in the White Group despite losing to Garbine Muguruza 3-6, 6-0, 6-1.
Kuznetsova will next play Dominika Cibulkova, who reached the semifinals despite finishing 1-2.
Pliskova offered Radwanska too many free points, making 29 unforced errors to only eight for the Pole.
Radwanska initially trailed 4-2 in the first set, but she soon recovered to 4-4.
"I just really play my best tennis against her," Radwanska said. "I can really play my game and do my things, things that I want to do on court and have a good rhythm."
In the second set, Radwanska scored the final service break in the eighth game when Pliskova netted two consecutive overheads.
Kuznetsova managed to win the first set against Muguruza, but looked fatigued after that.
"Definitely, you don't want to lose the match," said Kuznetsova, the last player to qualify for the season-ending tournament. "You try to fight. Sometimes the lack of energy just won't let you do that."