The 37-year-old, who initially retired in 2012, made her second comeback to the court last year.
Clijsters, however, was forced to undergo knee surgery in October and then her troubles worsened in January when she contracted Covid-19.
The Belgian, though, is not planning to retire again just yet and has vowed to keep working to try and make a comeback.
She wrote on Twitter: “In October, I had surgery on my right knee to repair damage sustained over my career and to reduce pain. Results following surgery were good – rehab and recovery went well and I was feeling really positive.
“As you’ve heard, I had COVID in January and while the symptoms were mild, it did disrupt my training schedule.
“I’ve had an intense few weeks of training with my team. 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 have been out of my control. I’m not ready to quit….. I’m going to keep pushing and see what’s possible.
“I want to thanks Miami and Charleston teams for the wild card opportunity and wish them all the best with their events.
“I am going to undergo some pain management treatment for the next 3-6 weeks and then regroup with my team and see what’s next.”