Kim Clijsters announced she will not play in Miami and Charleston in the coming months because of health reasons, further delaying her start to 2021.
The Belgian former World No.1 underwent right knee surgery in October and contracted COVID-19 in January.
"Results following surgery were good," Clijsters wrote on social media. "Rehab and recovery went well and I was feeling really positive. ... While the [COVID-19] symptoms were mild, it did disrupt my training schedule.
"Unfortunately I am not where I need to be, especially if I want to compete with the best."
Clijsters' second comeback began one year ago, but the tour was paused just three weeks later. The 37-year-old mother of three was only able to play three matches in 2020 - first-round losses to Garbiñe Muguruza in Dubai, Johanna Konta in Monterrey and Ekaterina Alexandrova at the US Open. Currently ranked World No.1055, Clijsters had been offered wildcards into Miami and Charleston this year.
Despite the setback, Clijsters said she has no plans to abandon the comeback.
"It's disappointing and frustrating," she wrote. "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."