The 21st-seeded Errani won for the second time in a row against a top-10 opponent after starting her career 0-28 in such matches. She beat No. 10 Angelique Kerber in the quarterfinals.
Errani never made it past the third round at any of her first 17 major tournaments, before getting to the quarterfinals at the Australian Open in January, then surpassing that at Roland Garros.
Errani faces No. 2 Maria Sharapova or No. 4 Petra Kvitova in Saturday's final.
Errani also will play in the women's doubles final with Roberta Vinci.