The Guernsey star, who will be replaced as British number one by Laura Robson next week, has complained about tiredness this season and had threatened to take a break in any event after admitting she felt 'burnt out.'
Now though it appears there was a medical reason for her slump in form and motivation.
"I had to do some blood tests after Miami and the results have shown that I have traces of Glandular Fever (mono) 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," said Watson.
"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."