Venus Williams wore a lacy white short-sleeve top, blue jeans and sandals. Her hair fell in soft curls past her shoulders, her dangly silver-dollar-size earrings shimmered in the light. In the restaurant of the Upper East Side hotel where Williams is staying, she could have passed for a model headed to the Hamptons for the Labor Day weekend rather than a tennis superstar already plotting her return despite a chronic autoimmune disease.

Sjogren’s syndrome, the disease, is difficult to diagnose because those who have it often appear the picture of health even when they do not have the energy to sit for a portrait.

“The fatigue is hard to explain unless you have it,” Williams said Thursday morning. “Some mornings I feel really sick, like when you don’t get a lot of sleep or you have a flu or cold. I always have some level of tiredness. And the more I tried to push through it, the tougher it got.”

Williams, 31, has won 10 tournaments in the last four years despite feeling fatigued and having difficulty breathing, symptoms she knows now are associated with Sjogren’s.