Serena Williams retired hurt from her Hopman Cup clash with Jarmila Wolfe on Tuesday, as the world number one's injury-hit start to 2016 continued.

The American pulled out of a match against Elina Svitolina on Monday after a practice session left her with an inflamed knee.

Williams looked sluggish in losing the first set against Wolfe 7-5 in Perth a day later and opted to withdraw after falling 2-1 down in the second set following a break of her serve. 

The reigning Australian Open champion, Williams is struggling to find fitness and form ahead of the defence of her title, with action in Melbourne getting under way on January 18.