"I've had to make the decision that I will not be able to compete in January," Clijsters tweeted. "I am undergoing rehab and treatment for a knee injury. It's a setback but I'm determined as ever to get back to the game I love." Clijsters won the US Open in 2005, 2009 and 2010, and the Australian Open in 2011.
She initially retired in 2007 aged 23, making a return two years later after having her first child to win her second US Open, then hanging up her racquet again after the 2012 US Open.
Clijsters made her WTA debut aged 15 at Antwerp in 1999. She has 41 singles titles, and is the only mother to top the WTA charts since computer rankings began in 1975.
Despite her lengthy time out she still ranks third among active players in terms of tournament success behind the Williams sisters - Serena with 72 titles and Venus with 49.