(Reuters) - Former world number one Kim Clijsters underwent knee surgery last year and contracted the novel coronavirus in January but the mother of three is not ready to quit just yet.

The 37-year-old Belgian has not managed a win since coming out of retirement for a second time in February, 2020, and her last match was in the opening round of last year's U.S. Open in September.

Clijsters was scheduled to return at the WTA 1000 event in Miami starting this month but has been forced to pull out.

"In October, I had surgery on my right knee to repair damage sustained over my career and to reduce pain," she said on social media. "Results following surgery were good - rehab and recovery went well and I was feeling really positive.

"I had COVID in January and while the 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, who won her first Grand Slam title at the 2005 U.S. Open, first retired from tennis in 2007, taking a break to have the first of her three children.

She returned to the sport in 2009 and won the U.S Open again as an unseeded wildcard in her third tournament back.

The Belgian won her third Flushing Meadows title in 2010 followed by an Australian Open the following season before retiring again in 2012.

"It's disappointing and frustrating," Clijsters said, adding that she will also miss next month's WTA 500 event in Charleston, United States.

"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."

(Reporting by Sudipto Ganguly in Mumbai; editing by Ed Osmond)