The Dane was leading 5-0 in the first set when sixth seed Radwanska called for the trainer at the changeover and decided to pull out of the $2 million tournament in Tokyo.
Wozniacki, winner of five titles this year and closing in on the world number one ranking, plays eighth seed Victoria Azarenka of Belarus in the semifinals. Azarenka beat American qualifier Coco Vandeweghe 6-2. 6-1.
French Open champion Francesca Schiavone of Italy beat Estonia's Kaia Kanepi 7-5, 4-6, 7-5 and will face either Elena Dementieva or Vera Zvonareva for a place in Sunday's final.
Verdasco exits
Second seed Fernando Verdasco was eliminated in his opening match of the Thailand Open after suffering a 6-4, 6-4 defeat by German Benjamin Becker on Thursday.
Seventh seed Viktor Troicki of Serbia was knocked out earlier on Thursday after a 7-6, 6-2 defeat at the hands of Finn Jarkko Nieminen, who saved 14 of 15 break points to stay alive in the tournament.
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