The injury woes of Serena Williams have continued, with the world No.1 pulling out of her Hopman Cup clash with Jarmila Wolfe on Tuesday night.

Williams was trailing 7-5 2-1 when she retired with a knee injury.

The 21-time grand slam winner also withdrew from Monday’s 2-1 loss to Ukraine because of inflammation in her left knee, with Vicky Duval taking her place in that tie.

Williams struggled to move freely against Wolfe, and she was especially vulnerable to any balls played to her left.