Williams started in blistering fashion against Ivanovic, romping through the opening set in ruthless fashion.
However, Ivanovic showed real grit as she got a foothold into the match and rattled off four games in a row at the start of the second set as the momentum swung her way.
Williams recovered her poise though and normal service was resumed in a one-sided third set.
In the other semi-final Errani became the first Italian to reach the final in nearly 30 years after she saw off Serbia's twice former champion Jelena Jankovic 6-3 7-5 in front of a passionate Foro Italico crowd.
"The crowd was a motivation for me, they were unbelievable," said a delighted Errani, who will have to beat Williams if she is to go one better than compatriot Raffaella Reggi in 1985.
- Sports & Recreation