After a skittish start, Williams dominated Errani and broke her final service game in both sets.
The defending champion improved to 5-0 against the seventh-ranked Errani and to 30-2 overall this year.
Williams will play the winner of the other semifinal between second-seeded Maria Sharapova and Ana Ivanovic. If Sharapova makes the final, the No. 1 ranking will also be at stake.
Later Saturday, Rafael Nadal played fellow Spaniard Pablo Andujar and Tomas Berdych faced Stanislas Wawrinka in the semifinals of the men's event.
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