Serena Williams has withdrawn from the Hopman Cup due to her troublesome knee injury.
The world number one retired hurt when 7-5 2-1 down in her Group A clash with Jarmila Wolfe in Perth on Tuesday, 24 hours after she was forced to pull out of her match against Elina Svitolina.
Williams has now opted to miss the rest of the tournament, and could face a race against time to be fit for the start of the Australian Open, which begins on January 18.
A short statement on the Hopman Cup's Twitter profile read: "Serena Williams has withdrawn from the #HopmanCup due to injury. Vicky Duval will replace her in the USA's final tie tomorrow night."