(CNN)It's been a disappointing season for two-time Wimbledon champion Petra Kvitova -- but the tennis star now has a reason for the lethargy that has plagued her.
Petra Kvitova: Two-time Wimbledon champ diagnosed with 'kissing disease'
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The world No. 4 has struggled to recapture the scintillating form she showed in trouncing Canadian starlet Eugenie Bouchard in the 2014 final at the All England Club.
Although triumphant at WTA Tour events in Sydney and Madrid in January and May respectively, Kvitova failed to make the second week at the Australian Open, Roland Garros or Wimbledon and was forced to pull out of events in Indian Wells and Miami due to fatigue.
Her preparations for the grass-court season were also curtailed by a sore throat, and her Wimbledon title defense ended in round three against Jelena Jankovic as Kvitova faded after winning the opening set.
Now the Czech star has revealed she has been diagnosed with mononucleosis -- a viral condition which can lead to prolonged fatigue, swollen glands, fever and body aches -- which could help explain her struggles with form and fitness.
"I had a sore throat and I wanted to kill it so I had antibiotics after Wimbledon," Kvitova said in comments carried by the WTA website.
"Unfortunately I didn't kill it and that's why it showed my immune system was low. So I had more tests and it (mononucleosis) showed up."
In worst-case scenarios, the effects of mono can linger for months or even years. It has earned the nickname of the "kissing disease" because it can get passed from one person to another through saliva.
Male players Robin Soderling and Mario Ancic were effectively forced to retire in their mid-20s after contracting the virus, though Roger Federer has recovered fully since being diagnosed with it in 2008.
On the women's tour, Christina McHale and Heather Watson reported suffering from mono in 2013.
Former tennis pro Justin Gimelstob told CNN in 2013 that tennis stars were susceptible to mono due to the intensity of travel and physical exertion involved on the tour, which can wear down the body and its immune system leaving it more vulnerable to infection.
That same year Andy Murray told UK newspaper the Daily Telegraph that he was acutely aware of contracting the illness due to the stresses of being on tour.
Kvitova said she was given the news last week and it's likely she has had the condition for months. However, she was keen not to use it as an excuse for her fatigue or sluggish form in the period since.
"It's tough when I'm looking back," the 25-year-old said. "Many things are clearer now. But of course I'm not sure if I don't have mono that I'm going to play well.
"That's not something I can say. I still had chances to win anyway. Maybe I can just see why I felt tired," she added.
Kvitova said she is still determined to compete in the North American hard-court season ahead of the U.S. Open, and try to qualify for the season-ending WTA Finals in Singapore.
Although past the contagious stage and clear to play at this week's Rogers Cup in Toronto, Kvitova says she still feels far from 100%.
"Of course I still feel tired and a little bit sleepy. I don't know if it's the jet lag or this thing.
"I don't think it would be right to tell everybody that I was practicing so hard after Wimbledon and (that) I'm so ready to play this tournament and win it.
"(But) I will try my best for sure. We are making the schedule smart with the practicing so I'm hoping it will be fine soon."