British No 1 Heather Watson is facing several weeks out having been diagnosed with glandular fever.
Blood tests in Miami revealed the 20-year-old is at the tail-end of the illness but experts are still unable to predict when she will be able to resume training.
Watson said: "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."
Iain Bates, LTA Head of Women's Tennis, 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."
Watson, who will surrender her British No 1 spot to Laura Robson next week, is hoping to improve on her third-round appearance at Wimbledon 2012 this summer.