Doctors have confirmed that world number one Justine Henin-Hardenne is suffering from a viral infection which she says is similar to glandular fever.
A medical report said her tiredness was due to a "CMV viral infection" and prescribed rest until the beginning of May.
She will be reassessed at the beginning of next month to see when she can start playing again, doctors at the Bois de l'Abbaye hospital, near Liège, said.
Henin-Hardenne is due to defend her French Open crown at the end of May. The claycourt Grand Slam runs from May 24 to June 6.
The 21-year-old Belgian withdrew from the Family Circle Cup in Charleston eight days ago saying that low blood sugar was causing light headedness.
She also pulled out of the Antwerp Diamond Games in February with a respiratory illness.