Caroline Wozniacki revealed Thursday at the WTA Finals that she was diagnosed with rheumatoid arthritis before the US Open.

"It turns out that I have an autoimmune disease, rheumatoid arthritis, which goes in and attacks your joints. When the body has a lot of fluids in it and you swell up, you get tired, you get exhausted, all these things.

"In the beginning, it was a shock, just, you feel like you're the fittest athlete out there, or that's in my head, that's what I'm known for, and all of a sudden you have this to work with," Wozniacki told reporters after losing 5-7, 7-5, 6-3 to Elina Svitolina, ending her season.

The 28-year-old Wozniacki, who said she began feeling symptoms of fatigue after Wimbledon, said she is taking medication to treat the autoimmune disease. She received the diagnosis in August after the Rogers Cup tournament in Montreal.

After Wimbledon in July, she thought she had the flu when on vacation. When she started playing again, her legs hurt. Then, in Montreal in August, she woke up one morning and couldn't lift her arms above her head. Wozniacki thought she may have glandular fever, but the doctor said she was fine. However, she was certain something was wrong and went for more in-depth testing.

Doctors told her that the disease should not affect her career, and she has recorded one tournament victory since the diagnosis: the China Open earlier this month.

"Obviously, winning in Beijing was huge. It also gave me the belief that nothing is going to set me back. I'm going to work with this, and this is how it is, and I can do anything," she told reporters.

She said she didn't want to discuss the diagnosis during the season but is comfortable doing so now that her season is over.

"I think I didn't want to talk about it, obviously, during the year because I don't want to give anyone the edge or thinking that I'm not feeling well. But I have been feeling well," Wozniacki told reporters. "You learn how to just cope after matches. Some days you wake up and you can't get out of bed and you just have to know that's how it is, but other days, you live and you're fine. You don't even feel like you have it.

"So, it's a lot. It's something that now I'm happy that I'm done with the season and you can just kind of control it a little bit more and figure out a plan how to control it even better in the future."

Wozniacki is No. 3 overall in the WTA rankings. She won the Australian Open earlier this year for her first Grand Slam title.

The Associated Press contributed to this report.