Perth - Serena Williams' Australian Open title defence is under a major injury cloud after she retired from her singles match against Australia Jarmila Wolfe at the Hopman Cup on Tuesday evening in Perth.
The world number one and winner of 21 Grand Slams will be aiming for a seventh singles title in Melbourne later this month, but is being troubled by a nagging left knee injury and retired when trailing 7-5, 2-1.
The problem sidelined her for much of the latter part of 2015 and she withdrew from the United States' opening Hopman Cup tie on Monday, citing inflammation of the left knee after a practice session.
Williams trained again briefly on Tuesday and took to the court against Wolfe (nee Gajdosova), but appeared very restricted in her movements especially on her left side.
The 34-year-old served for the first set, but was broken by Wolfe, who then took the set 7-5.
When Wolfe broke her again for a 2-1 lead in the second set, Williams called for the trainer and after a brief discussion retired.
Williams' coach Patrick Mouratoglou conceded in November that her knees were troubling her.
She did not play for two months after her calendar Grand Slam hopes were derailed by Italian Roberta Vinci in the US Open semi-finals in September.
Williams returned in the International Premier Tennis League in Manila last month.