The World number one beat Croatian Donna Vekic 6-3 4-6 7-6 (7-4) at the Porsche Tennis Grand Prix.
Osaka will now face Anett Kontaveit, who was able to progress after Victoria Azarenka retired.
Petra Kvitova will face Kiki Bertens in the other semi-final.

Real Sociedad beat Atletico Madrid on penalties to win Copa del Rey
Cunha strike earns Man United hard-fought 1-0 win at Chelsea
One driver dead, six injured in Nuerburgring race crash
Brighton's Rutter denies Tottenham crucial victory with last-gasp strike
France's Pin becomes first woman to drive a Mercedes F1 car
