The No.9-seeded Radwanska was the first to advance, needing just 65 minutes to dominate former Wimbledon semifinalist Tsvetana Pironkova, 6-2, 6-2. Radwanska's serving was particularly impressive - she never faced a break point in the match and lost just eight points in eight service games.
Radwanska has beaten Stephens in all three previous meetings, though none of them have been easy.
The No.2-seeded Wozniacki followed Radwanska into the final four in the last match of the day, squeaking out a marathon 59-minute first set then steamrolling No.10 seed Andrea Petkovic, 7-5, 6-1.
"The first set was pretty tight. I thought we were both playing pretty good tennis," Wozniacki said. "But I think I especially stepped it up in the second set, took the ball a little bit earlier and got some of those service games that I wasn't getting in the first set - she was returning well today, so it was not easy.
"I love playing here. It's one of my favorite events. It's always low key, it's fun, big crowds - the crowd is always nice and cheering all of us on. And it's kind of nice to have that calm before the storm."
Awaiting Wozniacki in the semifinals will be another WTA Rising Star, Belinda Bencic, who rallied from a set down to end the career-best run of British wildcard Johanna Konta in three sets, 2-6, 6-0, 6-3.
"Obviously I played a very solid, good first set, and started the match really well," Konta said. "She definitely stepped up her level after that. She played very well. All credit to her - she played tough and solid when she needed to, and she just played the situation - she was definitely the better player today.
"But I'm really happy with my effort this week. I'm really happy with the week in general. Today I just played against a player who was very tough - she stayed tough, and she came out on top in the end."
Wozniacki and Bencic are tied in their head-to-head, 1-1. This will be their first meeting on grass.
A big thank you to @BritishTennis for giving me the opportunities to compete in the main draw of such great events these past 3 weeks.
Aaaaand a huuuuuge thank you to all the home crowd support this week in Eastbourne! You guys made my experience magical.