Caroline Wozniacki, her tennis season over after a loss at the WTA Finals on Thursday, revealed that she had recently received a diagnosis of rheumatoid arthritis, an autoimmune disease that can cause joint pain and stiffness and problems with mobility.

“In the beginning, it was a shock,” Wozniacki, 28, said at a news conference in Singapore, the site of the WTA Finals. “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, a two-time United States Open finalist from Denmark, returned to No. 1 in the world in January after winning her first Grand Slam singles title at the Australian Open with a grueling three-set victory over Simona Halep.

But her results in the year’s other major events were not so impressive, and she said she had begun experiencing fatigue and joint soreness after losing in the second round at Wimbledon.