Petra Kvitova tells press in Toronto that she was diagnosed with mononucleosis following Wimbledon. The two-time Wimbledon champion, who lost to Jelena Jankovic in the third round of this year's tournament, also pulled out of Indian Wells and Miami earlier this year due to exhaustion.

“I had a sore throat and I wanted to kill it so I had antibiotics after Wimbledon,” said Kvitova. “Unfortunately I didn't kill it and that's why it showed my immune system was low. So I had more tests and it showed up. I found out I was diagnosed with mono. So it's going to be a little tougher for me to practice in a good way.”

Kvitova said that her doctor has cleared her to play, and she is competing this week in Toronto. It was indicated she caught the virus in the spring.

When she returned to the tour in April, the Czech won both of her singles matches against France in Fed Cup, then lost in the first round at Stuttgart. She went on to win Madrid, during which she handed Serena Williams her only loss of the season so far, then reached the quarterfinals in Rome and the fourth round at the French Open.

She said she didn’t feel physically great at the time, and now knows the cause. “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,” she said. “That's not something I can say. I still had chances to win anyway. Maybe I can just see why I felt tired.

"At first I was a little bit scared," Kvitova added. "I didn't know if I could play the U.S. tournaments this summer or maybe if I could play at all for the rest of the season. On the other side I think it's positive news. I know now why I'm still tired and feeling so badly. That's the good news for me.”

Last summer, the 25-year-old fell lost to Ekaterina Makarova in the second round of the Rogers Cup in Canada, then lost to Elina Svitolina in the opening round in Cincinnati. She managed to win New Haven, but then was stunned by Aleksandra Krunic in the third round at the U.S. Open.

Kvitova isn’t sure that she can play her best at the moment, but wants to compete.

“Of course I still feel tired and a little bit sleepy,” she said, “I don't know if it's the jet lag or this thing. 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.”