The Russian claimed glory in the WTA Premier event in each of the last two years, and Friday's quarter-final victory means she will remain in the top 10 of the world rankings, where she has been since April 2011.
Sharapova got off to a flying start on the clay in Germany, breaking her opponent in the very first game.
However, Radwanska broke back immediately and the set followed that pattern until Sharapova broke for a fourth time to make it 4-3.
She saved three break points in game eight to secure the crucial hold, before serving out in game 10 to edge in front.
The second set belonged to Sharapova, who raced into a 4-0 lead, and although Radwanska saved six match points in game eight, the number one seed could not stop her opponent from reaching the last four.
Sharapova will face Sara Errani, who progressed after her opponent Carla Suarez Navarro retired in the final set with the Italian leading 6-3 6-7 (6-8) 3-0.
Ninth seed Ana Ivanovic also progressed, knocking out 2009 champion Svetlana Kuznetsova 6-3 2-6 6-4.
Kuznetsova broke the Serb in the opening game, before Ivanovic hit back with breaks of her own in the fourth and the sixth to move in front.
The Russian hit back in the second, breaking Ivanovic three times on her way to levelling the match.
The decider was a tense affair, the first eight games going on serve before Ivanovic made the crucial break in the ninth to set up the victory and a place in the semi-finals.
She will face fellow Serb Jelena Jankovic, who overcame Alisa Kleybanova 6-4 6-3.
Fifth seed Jankovic broke her opponent in the very first game, and held her nerve throughout the remainder of the set to get her nose in front.
Jankovic, who won the title in 2008, got the better of her opponent in a second set that saw six breaks of serve, eventually clinching victory when Kleybanova sent a volley long.