Serena Williams and Maria Sharapova have fitness concerns ahead of this month's Australian Open after both pulled out of warm-up events.
World number one Williams was troubled by a nagging knee injury and retired when trailing 7-5, 2-1 to Australia's Jarmila Wolfe in the Hopman Cup.
Five-time Grand Slam winner Sharapova has withdrawn from the Brisbane International with an arm injury.
World number two Simona Halep has also pulled out with an inflamed Achilles.
The Australian Open begins on 18 January.
Williams, who will be bidding to retain her title in Melbourne, has a recurrence of a problem that sidelined her for much of the latter part of 2015.