In the five matches she has played in the tournament, Williams has lost an average of 1.7 games per set. She needed just 58 minutes to defeat Ivanovic, recording 26 winners to nine for her opponent. Williams had 12 aces and Ivanovic had none.
Errani earned the unenviable task of taking on Williams by eliminating doubles partner Roberta Vinci in the quarterfinals 6-2, 6-4.
The other semifinal will send No. 3 seed Maria Sharapova against world No. 1 Victoria Azarenka.
Sharapova, who trailed 0-4 in the first set Tuesday when her match was suspended by rain, returned Wednesday and defeated Marion Bartoli 3-6, 6-3, 6-4. Sharapova broke for a 5-4 lead in the final set and closed out the match on her serve in the next game.
She set up her first match point with an ace and claimed the win when Bartoli was wide on a final return.
Azarenka reached the semifinals Tuesday by defeating defending champion Samantha Stosur.
Williams has won 24 of her last 25 matches, a stretch that has seen her claim the title in Stanford, Calif., in addition to her wins at Wimbledon and the Olympics.
Her only loss during that period came in the quarterfinals at Cincinnati in a 6-4, 6-4 setback to Angelique Kerber.
Williams will be playing in the semifinals of a Grand Slam tournament for the 21st time. She has lost in the semifinals just three times, the most recent occasion being in this tournament in 2009.