Former world No.1 Kim Clijsters has been warned she could struggle to compete with the top players when she returns to the WTA Tour in 2020 as the sport has become more physically demanding.

The popular Belgian won four grand slam titles before retiring from the sport for a second time in 2012 but in September the 36-year-old announced that she would come out of retirement again because of her love for the sport after eight years out.

“It’s hard to say what I expect from Kim as I don’t know how she is physically. Tennis really got way more physical the last few years on the WTA Tour,” Naomi Osaka’s former coach Sascha Bajin told the WTA website.

“For sure, she can still turn matches with her championship mindset and playing the right ball in big moments, but I really don’t know if that’s going to be enough to compete in this modern age.”

Clijsters, who first retired in 2007 to start a family, had planned to resume her career in January but then delayed her comeback after suffering a knee injury.

Bajin, who guided Osaka to two grand slam titles and was named 2018 WTA coach of the year, believes Clijsters has the advantage of experience if she decides to chase a fifth singles grand slam title.

“The biggest challenge is going to be competing again on a daily basis… bringing it every day,” he added.

“I think I want to say yes she can win another slam but at the same time I’m not sure… it’s not going be easy but if she’s coming back she can have great success.”