Caroline Wozniacki has spoken of her 'shock' after being diagnosed with rheumatoid arthritis earlier this year after her defeat to Elina Svitolina at the WTA Finals in Singapore.
The world No. 3 said she woke up one morning after playing at Wimbledon and was unable to lift her hands above her head.
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Having struggled with fatigue before the US Open, she was diagnosed with rheumatoid arthritis, a disease that limits mobility through painful joint inflammation and stiffness.
"In the beginning, it was a shock," world No. 3 Wozniacki told reporters after her 5-7 7-5 6-3 defeat by Ukraine's Svitolina on Thursday.
"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.
"It is what it is, and you just have to be positive and work with it, and there are ways that you can feel better so that's great."
Wozniacki had held the world No. 1 spot for close to 70 weeks before she won her first grand slam title at the Australian Open earlier this year. Victory in Melbourne again lifted the 28-year-old to the top of the world rankings.
She won her third title of the season in Beijing several weeks after her diagnosis, a victory she said "meant a lot to her" and boosted her confidence.
"I think you obviously start asking yourself questions, what does this mean, does it mean I can't get in as great of shape as I was before?" Wozniacki continued.
"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."
While rheumatoid arthritis is traditionally thought to be a disease suffered by older generations, recent studies have revealed that it is more prevalent in young adults. Of the 54.4 million American sufferers, 32.2 million were aged under 65.
Wozniacki has been receiving treatment and is on medication since her initial diagnosis in August and is hopeful she'll be able to manage the disease alongside her career.
"Some people can go into remission and some people, it just stops, the disease, and it's just right there and it's not going to get worse, or if it does, it's slowly.
"The medicine now is so amazing so I'm not worried about it. So that's great. You just have to be aware."
The year-end WTA Finals see the top eight players in the world go head-to-head for a $7 million prize.
Despite a victory against Petra Kvitova, defeats to Svitolina and Karolina Pliskova mean Wozniacki failed to qualify for the semifinals.