British number one Heather Watson has been ordered by doctors to rest after she was diagnosed with glandular fever.
The 20-year-old, who reached the third round of the Australian Open in January, has struggled for form this season and lost in the first round of the recent tournaments in Indian Wells and Miami.
The star announced she was taking a break from the game recently following her defeat by Ayumi Morita in Miami - in a match she had led comfortably - before reversing the decision 24 hours later, claiming she was simply been disappointed at losing the match.
But she headed home to Guernsey and pulled out of this week's tournament in Charleston, and blood tests have confirmed Watson is suffering from glandular fever.
She told the Lawn Tennis Association: "I had to do some blood tests after Miami and the results have shown that I have traces of glandular fever in my system.
"There is no prescription for this other than complete rest but the good news is that the doctors have told me I am at the tail-end of the fever. This means I have been trying to compete through the symptoms for some time."
Glandular fever is reasonably common in tennis players. Roger Federer suffered from it in 2008 while Justine Henin and Andy Roddick also got through it and resumed their careers relatively quickly.
But others have not been so fortunate. Mario Ancic was never the same player after contracting the disease while two-time French Open finalist Robin Soderling admits it may have ended his career at 26.
Watson is relieved to have a diagnosis, and said on www.lta.org.uk: "I have been struggling with both training and playing matches and felt so tired and exhausted for months but now that the doctors have found the cause of this it explains everything that has been happening to me physically and mentally.
"I am actually relieved to know what is wrong so that I can get myself on the mend. I hope to recover as soon as possible by resting my body and getting lots of sleep.
"I already feel better from being at home for a week and am looking forward to getting back on tour soon."
When Watson returns will depend on her recovery but it now seems very unlikely she will be available for Great Britain's Fed Cup tie in Argentina later this month.
The world number 40's absence would be a major blow for Judy Murray's team as they aim for promotion to World Group II.
Iain Bates, head of women's tennis at the Lawn Tennis Association, said: "First of all, get well soon, Heather, from the whole team.
"We all know she'll bounce back better than ever. Heather's focus will be on her recovery and we will do whatever we can to support Heather and help that process."