The 2010 champion had originally chosen the event as the potential start to her return to the WTA Tour as a mother, but decided to begin her comeback on grass courts. The Belarussian will now have to delay her hard-court return a little further.
“I’ve been sick with a viral illness all week, and will therefore not be able to compete next week,” she said in a statement.
Maria Sharapova and Petra Kvitova are among the leading names in the field for Stanford, along with top seed Garbine Muguruza.
Azarenka is still scheduled to compete the following week in Toronto, and is playing with a protected ranking as she returns to the WTA Tour.