British number one Heather Watson has been diagnosed with glandular fever.
Blood tests have revealed traces of the virus, although doctors say she is at the tail-end of the fever.
Watson said: "I am relieved to know what is wrong so that I can get myself on the mend."
Watson, who reached the third round of the Australian Open in January, lost in the first round in Indian Wells and Miami.
"After an impressive rise up the rankings to
the brink of a Grand Slam seeding, Watson
has struggled in recent weeks. Her last four
defeats have been to players ranked below
her. She talked about a curious lack of
energy in matches at the Australian Open
two months ago and now we know why.
She has felt exhausted for months with blood
tests showing she's at the tail end of a bout
of glandular fever. While there's no news on
how long her recovery will take, she has
been advised to rest completely. Glandular
fever has the ability to hit athletes hard. The
Swedish top 10 player Robin Soderling
hasn't played for almost two years after
contracting a particularly vicious strain of
the virus. Everyone in tennis wishes Heather
well and a much speedier return."
The Briton then pulled out of this week's WTA tournament in Charleston, South Carolina, with a leg injury and returned home to recover.
"I had to do some blood tests after Miami and the results have shown that I have traces of Glandular Fever in my system," she added.
"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.
"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 hope to recover as soon as possible by resting my body and getting lots of sleep."
Professor Paul Farrell, who teaches tumour virology at Imperial College London, said recovery from glandular fever can vary.
"Some people literally recover in two weeks but some people take four to five to six months, although that is very unusual," said Farrell.
"If she has been clinically diagnosed with this disease she should make a proper recovery. In all likeliness she will recover and be back to perfectly normal within weeks to months."