Swedish tennis star Robin Soderling, the former world No.4 sidelined by illness for almost two years, still hopes to return to the circuit.

“I will come back when I feel 100 per cent but I don’t know when. That’s how it is, unfortunately,” he said in an interview published on Friday by the Aftonbladet daily.

Soderling – the only man to have beaten Spain’s eight-time French Open champion Rafael Nadal on the clay at Roland Garros – has not played competitively since the Swedish Open in July 2011.

Since then, he has been hit by a series of health problems, including glandular fever, which is still affecting him.

The 28-year-old, who turned professional in 2001, told the newspaper that he was not yet fit enough to endure the rigours of competition tennis at the highest level.

“It’s hard to recover at a snail’s pace,” he explained.

“In the beginning, it was difficult. When I went to bed every night, I told myself: perhaps I’ll be better tomorrow.

“That lasted a week, a month … I got another perspective on the passage of time.”

The Swede said he had made enough money during his career so did not have to worry about working again.

According to the atpworldtour.com website, he won more than $US10.4 million ($A10.83 million) in prize money in singles and doubles.

“It’s an extraordinary feeling to be able to choose what you do, rather than having to do it. I’ve been spoiled,” he added.