The tennis champ, who has had her own health scares recently, tells PEOPLE her sister's diagnosis made her realize that life "isn't all about tennis."
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"I love playing tennis and I love the battle," she says, "but I realize that life is so precious."
Serena adds that Venus is in good spirits and she's staying positive.
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Sjorgen's Syndrome is an autoimmune disease that causes fatigue and joint pain. Some four millions Americans suffer from the condition.
Get well, Venus!