Perth, Australia - Serena Williams is booked for a series of three exhibitions this week in the United States, but that fact has not kept the world number two from withdrawing as defending champion at next month's Hopman Cup.
Williams and James Blake had been due to defend their title at the eight-nation mixed team competition starting January 3.
But Williams, claiming a hamstring injury, said she would be unable to participate. Her place in the team of five-time champion US will be taken by world number 50 Meghann Shaughnessy.