Four-time Grand Slam winner Kim Clijsters has withdrawn from the Miami Open this month and the Charleston Open in April.
Her return to tennis in 2019 was halted by a medial ligament tear. She returned just as the Covid-19 pandemic hit March 2020.
Her comeback was then halted further by knee surgery last October and getting Covid-19 three months ago.
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"I've had an intensive few weeks of training with my team," the 37-year-old wrote in a statement put out on Twitter.
"Unfortunately I am not where I need to be especially if I want to compete with the best.
It's disappointing and frustrating. I knew this was going to be hard when I started this journey - and there have been challenges that are out of my control. I'm not ready to quit... I'm going to keep pushing and see what's possible.
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