PERTH, Australia -- Serena Williams withdrew from her opening match at the Hopman Cup on Monday because of inflammation in her left knee, an early setback in her preparations for an Australian Open title defense.
The top-ranked Williams practiced ahead of her scheduled season-opener against Elina Svitolina of Ukraine in the international mixed-team competition at Perth Arena, but withdrew from the match and was replaced by Vicky Duval.
Williams said she expected to be able to join Sock in the U.S. team to play the Australia Gold team on Tuesday.
Hopman Cup tournament director Paul Kilderry said Williams reported feeling pain in her knee on Sunday.
"She wanted to give herself every chance to play, she warmed up this morning but common sense prevailed and she just thought today she can't do it," Kilderry said.
After winning the Australian Open, French Open and Wimbledon titles in 2015, Williams narrowly missed completing a season Grand Slam when she lost to Roberta Vinci in the semifinals of the US Open.
For the 20-year-old Duval, who returned to tennis in August after overcoming Hodgkin's lymphoma, filling in for Williams was a great opportunity.
"I was kind of nervous because I was filling in big shoes. But I had a really fun time today," Duval said. "I have a whole new outlook on life. I'm just really grateful to be back and do what I love."
Two-time Australian Open champion Victoria Azarenka returned from a layoff with foot and leg injuries with a 6-2, 6-0 win over Russian qualifier Elena Vesnina in her first competitive match since September, setting up a second-round match against No. 2-ranked Simona Halep.
In other women's singles, No. 6-seeded Carla Suarez Navarro advanced to a second-round match with local favorite Sam Stosur, the former US Open champion who recovered from early service breaks in each set to beat Slovakian qualifier Jana Cepelova 6-4, 3-6, 6-4.
At the ASB Classic in Auckland, New Zealand, fourth-seeded Svetlana Kuznetsova progressed to the second round, leading 6-2, 1-1 when Mona Barthel of Germany retired from their first-round match because of a stomach ailment.