Hobbling Serena ruled out of opening Hopman Cup tie

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.

(Reuters)

"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.

(Reporting by Nick Mulvenney, editing by John O'Brien)