The crafty Pole, a former champion in the sunshine state, netted just her second victory over a reigning world No.1 as she rallied from a set down to eliminate the top seed 3-6 6-2 6-3.
Radwanska has endured a tough time with injuries recently and her ranking has suffered as a consequence, but her latest performance suggests her career is moving forward again.
It’s a missed opportunity for Halep. Caroline Wozniacki’s second round loss to Monica Puig provided the Romanian with the chance to strengthen her grip on top spot, however she has been unable to make the most of a promising situation.
Elsewhere on the grounds, Victoria Azarenka, a wildcard entry, overcame Anastasija Sevastova 3-6 6-4 6-1 to make it three wins in succession for the first time since Wimbledon.
“It was a difficult match,” said the Belarusian, a three-time winner in Miami. “Obviously, I don’t feel like I played my best tennis, but these kind of matches are super important for me right now, to be able to find a way to go through.
“I have to build on from this, take confidence from this match, that I was able to figure it out.”
Azarenka reward is a fourth round encounter with Radwanska, who she has beaten in 12 of their 17 previous meetings.
Former world No.1, Karolina Pliskova, continues to made steady progress. The Czech edged a marathon scrap with Su-Wei Hsieh 6-4 1-6 7-6(4) to reach the last sixteen. Garbine Muguruza, Angelique Kerber, Yafan Wang and Zarina Diyas are also through to the second week.