World number one Serena Williams was ruled out of her opening match at the Hopman Cup mixed team event in Perth on Monday because of inflammation of the knee, organisers said.
The 34-year-old was scheduled to play her first official match since the U.S. Open last September for the United States in their round robin tie against Ukraine but pulled out after a morning warm-up at the venue.
"I'm disappointed to not take the court in Perth today," Williams said in a news release.
"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."
Williams, who is using the event as preparation for her tilt at a 22nd grand slam singles title at the Australian Open later this month, will be replaced in the U.S. team by 20-year-old Vicky Duval.