PERTH, Australia -- Serena Williams withdrew from the Hopman Cup on Wednesday because of inflammation in her left knee, a setback in her preparations for the Australian Open.
Williams has been replaced by Victoria Duval in the U.S. team for the last Group A match against the Czech Republic on Thursday. Williams withdrew ahead of her season-opening match against the Ukraine on Monday, with Duval filling in for her, and retired in the second set against Australia's Jarmila Wolfe on Tuesday.
Hopman Cup organizers confirmed Williams' withdrawal on Wednesday. The U.S. team is 0-2 and cannot reach the final of the international mixed team competition at Perth Arena.
Williams, who won the Australian Open, French Open and Wimbledon and got within two victories of a season Grand Slam in 2015 before losing in the US Open semifinals, had initially been confident of returning to action in Perth.
"It's not even a bump -- just a really minor thing in the road, and I'll fly over it," Williams said after retiring against Wolfe. "I just have some inflammation that's been going away very slowly -- it's going away, but it needs a little more time.
"Mainly I was afraid to move, and I need to get in the space where I'm not afraid to move. Maybe a day off or two will really make the world of difference."
Williams is expected to travel to Melbourne to continue preparations for the Australian Open, which begins on Jan. 18.