Prague: Czech star Petra Kvitova`s recent drop in form has been blamed on the world number four suffering from mononucleosis earlier this year.
"Blood tests showed that she had mononucleosis in the spring," the 25-year-old`s spokesman Karel Tejkal told AFP on Monday.
The 2011 and 2014 Wimbledon champion and three-time Fed Cup winner will compete this week in the WTA tournament in Toronto, one of several tune-up events for the US Open starting later this month.
"Petra can continue her planned programme but under medical supervision," said Tejkal.
Kvitova mysteriously disappeared from the circuit at the end of February citing the "need to rest".
She returned in April for the semi-finals of the Fed Cup which the Czech Republic won 3-1 by beating France.
But the defending champion was beaten in the third round of Wimbledon by Serb Jelena Jankovic.
"Very probably the mononucleosis was the principle cause of her tiredness," added Tejkal.
A infectious virus mononucleosis can result in high fever, great fatigue and a sore throat but in some cames can have no apparent symptoms.
Kvitova won two titles -- Sydney and Madrid -- this season but only reached the third round of the Australian Open and Wimbledon and fourth round at Roland Garros.