Serena Williams has withdrawn from the Hopman Cup because of her troublesome knee injury.
Williams, the world's top-ranked women's tennis player, quit Tuesday's match when down 7-5, 2-1, to Jarmila Wolfe of Australia. The move came 24 hours after she pulled out of her match against Elina Svitolina.
MORE: Classic Serena images | Maria Sharapova ready | Garbine Muguruza confident
Williams has opted to miss the rest of the tournament and could face a race against time to be fit for the Australian Open, which begins Jan. 18 (Jan. 17 in the United States).
A short statement on the Hopman Cup's Twitter feed read: "Serena Williams has withdrawn from the #HopmanCup due to injury. Vicky Duval will replace her in the USA's final tie tomorrow night."