Pennetta, the top seed, beat Romania's Ioana Raluca Olaru 7-5, 6-2.
"It's a very long season and we're all very tired," a weary Pennetta said on the way to her fourth semifinal appearance this season. "I'm hoping to keep playing well here and at Moscow next week."
The second-seeded Radwanska, last week's runner-up at the China Open, was a 6-3, 7-6 (11-9) winner over seventh-seeded Lucie Safarova of the Czech Republic.
Also Friday, Belgium's Yanina Wickmayer, the No. 3 seed, topped eighth-seeded Sara Errani of Italy 7-5 6-3; and unseeded Petra Kvitova of the Czech Republic knocked off Carla Suarez Navarro, the fourth seed, 7-5, 6-4.
Saturday semis are Pennetta- Wickmayer and Radwanska-Kvitova.
Sunday's champion will earn $37,000.