Serena Williams out with knee, plans on Australian Open

World No. 1 Serena Williams was forced to retire with a left knee injury during the United States' 3-0 Hopman Cup loss to Australia Gold on Tuesday.

Williams missed the United States' opening Hopman Cup tie on Monday due to inflammation in her left knee. On Tuesday, Williams appeared restricted in her movements.

"I just have some inflammation that's been going away very slowly,'' Williams said. "It's going away, but just needs a little more time. I've been training really hard during the offseason. Really pushing myself beyond the limits. I just think a day off or two will make a world of difference.''

The injury could cast doubt over her title defense in Melbourne, where she will aim for a seventh singles title later this month. Williams insists she will be ready to go.