Serena (left) and Venus Williams
09/05/2011 AT 03:00 PM EDT
After Venus Williams withdrew from the U.S. Open
on Wednesday and revealed she was suffering from the autoimmune disorder Sjögren's syndrome, younger sister Serena Williams says Venus's condition is improving and she's staying positive.
"She's doing better," Serena, 29, told PEOPLE Saturday night after defeating Belarus star Victoria Azarenka at the U.S. Open in New York. "It's a day at a time kind of a thing. Her spirits are good, however."
Some 4 million Americans suffer Sjögren's syndrome, which causes fatigue and joint pain and would affect the 31-year-old Venus's tennis future.
"Tennis is great, but [Venus's illness] puts everything in perspective," says Serena, who herself suffered from a pulmonary embolism
and underwent an emergency treatment
for a blood clot in one of her lungs last spring. "I love playing tennis and I love the battle, but I realize that life is so precious and it isn't all about tennis."
In addition to praying and thinking constantly about her sister, what else is the 13-time Grand Slam champion doing to help her older sibling?
"I'm not the best with that," Serena says, "I'm a spoiled littler sister. She always takes care of me."
Having been inactive the past 12 months due to her own health condition, Serena is currently ranked 28th at the Open and has her next match Monday. But winning her fourth U.S. Open Women's singles title would mean a lot to her sister.
"She would definitely be happy," says Serena. "It would be really exciting for her and make her feel better."