Meanwhile, top-seeded Garbine Muguruza crashed out, meaning Saturday's second semi will be between Japanese wild card Naomi Osaka and the Ukraine's Elina Svitolina.
Second-seeded Radwanska, of Poland, hit eight aces in overpowering the Puerto Rican 6-2, 6-3.
''At this level, you have to play well to win by that kind of score,'' said Radwanska, who also won here in 2011.
''I knew Monica was in great form, winning the gold, but you just have to go on court and play your game. I was serving great and that helped me a lot.''
Over nine years, Wozniacki has an 8-4 record against Radwanska, and won their last three matchups.
''We've played so many matches everywhere on every surface, it's always a great challenge against her.,'' Radwanska said. ''For me, she's still a top-10 player, even though her ranking dropped a bit. Hopefully, I can serve just as well against her.''
Wozniacki, world-ranked 28, appears back near her best after ankle, knee, and bicep injuries; she reached the U.S. Open semifinals and continued that form.
She beat Polish qualifier Magda Linette 6-4, 6-3. Wozniacki won 79 percent of points on her first serve, including 90 percent in the first set.
''I enjoyed the rallies, being able to get into a rhythm, and having a good time - it's obviously more fun when you're winning,'' Wozniacki said.
Osaka came from 5-0 down in the second set against Belarussian qualifier Aliaksandra Sasnovich to seal a 6-3, 7-6 (6) win and reach a first WTA semifinal.
Osaka overcame a bout of teen tantrums, twice smashing her racket down, and ease off her big serve, to retrieve the second set.
''It was fun to play, I've just got to work on my attitude a little bit,'' she said. ''I was focusing more on playing the shots instead of the result, at 0-5. I felt she had a great backhand return, so I thought I would mix it up a bit and see what she did. Then I had the confidence to go for my serve again.''
Svitolina took the opening five games on the way to winning a two-hour fight against French Open champion Muguruza, 6-2, 4-6, 6-3.
''I don't think she missed a ball in that first set, and after that she was very comfortable,'' Muguruza said. ''I was really impressed with her movement, she covered everything.''