Serena Williams retired from her Hopman Cup match Tuesday against Jarmila Wolfe, as the world No. 1's injury-marred start to 2016 continued.
The American pulled out of a match Monday against Elina Svitolina after a practice session left her with an inflamed knee.
Williams looked sluggish in losing the first set against Wolfe, 7-5, in Perth, Australia, a day later and opted to withdraw after falling behind 2-1 in the second set after a break of her serve.
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"I have some knee inflammation that's going away very slowly. It's going, it just needs a little more time," Serena said. "I'm not making any decisions. I'm just taking it one day at a time."
The reigning Australian Open champion, Williams is struggling to find fitness and form ahead of the defense of her title, with action in Melbourne getting under way Jan. 18.