United States captain Mary Joe Fernandez told the Associated Press that Williams' withdrawal was not as a result of injury or illness.
It has been confirmed that Williams will be replaced by Lauren Davis, who is ranked 57th
In addition to Fernandez and Davis, the United States team consists of world number one Serena Williams, Alison Riske and Christina McHale.
The Italian team, which includes Sara Errani, Flavia Pennetta, Camila Giorgi and Karin Knapp, will be without Roberta Vinci who is recovering from an inflamed right elbow.
The best-of-five tie will be held at the Circolo Tennis Brindisi in Apulia.