Former world number one Kim Clijsters, who was due to return to competition in 10 days in Miami, pulled out of the tournament on Sunday but said on Twitter that she was "not ready to quit".

The 37-year-old Belgian, who won the Miami tournament in 2005 and 2010, had been handed a wild card to take part in her first official match since September 2 when she lost to Ekaterina Alexandrova in the first round at the US Open.

The four-time major winner underwent a knee operation in October and then contracted Covid-19 in January, forcing her to revise her plans.

"While the symptoms were mild it did disrupt my training schedule," the mother of three said 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 outside my control.

"I'm not ready to quit... I'm going to keep pushing and see what's possible."

Clijsters only played three tournaments and three matches in 2020, beaten each time in the first round.

Currently ranked at 1,055 in the world, Clijsters had also been offered a wildcard at Charleston.

She has 43 singles titles, including three US Open titles and one Australian Open.

She has also won the WTA Championships three times as well as the Fed Cup with her compatriot Justine Henin.