The four-time Grand Slam winner laid off for the first time in 2007, only 23 years old. Injuries were a big part of that decision.

She then made a comeback just over two years later and won the US Open in 2009. She played until 2012 before retiring.

Since then, Clijsters has become the mother of three children - at the same time she has seen other moms, such as Victoria Azarenka and Serena Williams, play top-level tennis. It has inspired her to come back.

- What she (Serena Williams) is doing is incredible, Clijsters has said before.

The plan was to return to the WTA tour as the season kicks off in January, but now Clijsters says a knee injury is changing her plans.

“I've had to make the decision not to play in January. I rehab and get treated for a knee injury. It's a setback, but I'm more determined than ever to get back to the game I love, "Clijsters writes on social media.