- Serena Williams pulls out of Hopman Cup due to knee injury
- Williams says she's been "pushing herself beyond the limits"
- Maria Sharapova and Simona Halep also suffer injuries
- Roger Federer to begin his 2016 campaign in Brisbane
(CNN)World No.1 Serena Williams has withdrawn from the Hopman Cup as she continues to manage a knee injury ahead of her Australian Open title defense.