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Appearing in her first final at this level, Errani got off to a bad star when Williams broke her serve straight away to move out to a 3-0 lead.
The Italian broke back in the seventh game to make it 4-3 but couldn't pull level as Williams re-established her advantage in the next game before serving out the set 6-3.
The second was a whitewash as Williams's power proved too hot for Errani to handle.
Errani won just five points in the set as top seed Williams cruised to victory in 73 minutes ahead of her title defence of the French Open in Paris, which begins on May 25.