Williams' nail art was on display during her matches on Wednesday and Friday. While most of the nails on her left hand were painted various pastel colors and bedazzled with jewels, the fingernail on her ring finger was decorated to look like a tiny koala bear.
An estimated 25,000 koalas have perished in the wildfires, which have been blazing since last September. The lives of more than 1 billion animals are believed to have been lost. Some animals are now feared extinct.
The big-hearted tennis star has repeatedly called attention to the disaster to help raise awareness about relief efforts. Earlier this month, Williams, who's won the Australian Open seven times in the past, shared photos of first responders rescuing injured koalas.
"I am heartbroken over the devastation of the wildfires in Australia," wrote Williams, adding, "I have been going to Australia for over 20 years so this devastation is hitting me particularly hard."
She also donated her $43,000 prize money to relief efforts from her win earlier this month at the ASB Classic Match in New Zealand. In the match, Williams defeated fellow American Jessica Pegula to win her first title since becoming a mom to her daughter, Olympia, in September 2017.
"What a start to the decade, @serenawilliams. We're all so proud of you," he gushed. "And just when I think I can't possibly be more proud, you donate all your prize money to fight the #australianbushfires."