document: Willett, 29, was due to partner Lee Westwood when the 72-hole tournament begins in Melbourne on 24 November.
  But the Masters champion does not want to risk aggravating the issue that saw him miss October's British Masters.
  "Considering everything, including the amount of travel involved and extra strain, I decided that it was not in my best interests," Willett said.
  The four-day competition would have been the fifth tournament in consecutive weeks for Willett, who has dropped out of the world top 10 for the first time since his Masters victory in April.
  Willett is second in the Race to Dubai and is tied for 54th in the penultimate tournament, the Nedbank Golf Challenge, after a round of 74 at Sun City.
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summary: England's Danny Willett has withdrawn from the World Cup in Australia to avoid aggravating his back problem.