Williams, 34, was scheduled to play Ukraine's Elina Svitolina in the women's singles on today morning in Perth, Australia, but she pulled out just minutes before the start. The match at the mixed-teams tournament was to launch Williams' preparation for her title defence at the Australian Open in Melbourne later this month.
     
Williams, who has won 21 Grand Slam singles titles, sat out the tail-end of last season saying she needed 'time to heal' after narrowly missing out on a rare calendar-year Grand Slam.
     
But she said the knee problem was only minor and hoped to be fit to play in the United States' second tie against Australia Gold, one of two teams representing the hosts, on tomorrow evening.
     
"I'm disappointed to not take the court in Perth today," Williams said. "I had every intention to play this morning.
     
"Unfortunately due to inflammation in my knee I need to rest and am confident to be out there against Aussie Gold tomorrow evening," she added.

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