Serena Williams of the United States looks on during her singles match against Jarmila Wolfe of Australia Gold during day three of the 2016 Hopman Cup at Perth Arena on January 5, 2016 in Perth, Australia.

Serena Williams talks with Player Services staff prior to withdrawing from her singles match against Jarmila Wolfe of Australia Gold with a knee in jury during day three of the 2016 Hopman Cup at Perth Arena on January 5, 2016 in Perth, Australia.

*Serena Williams experienced soreness in her left knee during the Hopman Cup on Tuesday in Perth, causing her to retire from the tournament – not to mention create a hurdle in her preparations for the Australian Open later this month.

Williams was forced to retire during the United States’ 3-0 loss to Australia Gold on Tuesday evening. She was in the midst of her singles match against Jarmila Wolfe, trailing 7-5, 2-1.

Serena Williams of the United States talks with Player Services staff prior to withdrawing from her singles match against Jarmila Wolfe of Australia Gold with a knee in jury during day three of the 2016 Hopman Cup at Perth Arena on January 5, 2016 in Perth, Australia.

Serena Williams of the United States talks with Player Services staff prior to withdrawing from her singles match against Jarmila Wolfe of Australia Gold with a knee in jury during day three of the 2016 Hopman Cup at Perth Arena on January 5, 2016 in Perth, Australia.

Veteran Lleyton Hewitt then secured the tie for Australia Gold with a 7-5, 6-4 win over Jack Sock, and it became a clean sweep for the home side when the mixed doubles was a walkover due to Williams’ absence.

The result meant the US bowed out of contention in the tournament. But of more concern to their camp was Serena’s readiness to defend her Australian Open title in Melbourne.

She had already missed the United States’ opening Hopman Cup tie on Monday due to inflammation of her left knee and was hoping to gain some valuable match practice against Wolfe. However, the 21-times Grand Slam singles champion was confident it was only a minor problem and even refused to rule herself out of Thursday’s Hopman Cup tie against the Czech Republic.

“I just have some inflammation that’s been going away very slowly,” she explained. “It’s still there, it’s going away, but just needs a little more time. A little rest, a little treatment. I’ve been training really hard during the off-season and really pushing myself beyond the limits, I just think a day off or two will make a world of difference.”

Williams will be going for her seventh Australian Open title, beginning Jan. 18.