World No. 1 Serena Williams is battling a knee injury and has withdrawn from the United States' Hopman Cup event Monday.
The 21-time grand slam champion was to face Elina Svitolina in Perth as the United States takes on Ukraine in the Group A opener.
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But an inflamed knee led to Williams being replaced by Victoria Duval, although the 34-year-old is confident of playing Tuesday.
"I had every intention to play this morning," she said.
"Unfortunately, due to inflammation in my knee, I need to rest. I am confident to be out there against Australia Gold (Tuesday) evening."
Williams is eyeing a seventh Australian Open title this month and her injury comes as a blow to her preparations, just two weeks out from the start of the year's first grand slam.