Williams will face Victoria Azarenka, whom she will replace at the top in Monday's new rankings update.
Azarenka beat Agnieszka Radwanska 6-3, 6-3 in the other semifinal.
Williams, who had been inconsistent for much of the week and struggled with her first serve, was dominant. She broke Sharapova to go up 4-2 and had a love service game on the way to winning the first set.
Sharapova came out stronger in the secondt and had a chance to go up 2-0. But Williams saved the break point and broke the third-ranked Russian to go up 2-1. She broke again for 5-2.