Billie Jean King toasts '80 years young' on milestone birthday

Billie Jean King is "80 years young" today, and the veritable firebrand says she's has no plans to slow down.

The tennis legend, equality advocate and WTA founder toasted her milestone birthday Wednesday with a heartfelt social media message, reflecting on where she's been in her life, who's been by her side, and how much she still can do.

"Today, I am 80 years young," King wrote. "Relationships are everything. I am fortunate to have been surrounded by loved ones all my life. I look forward to each day. Having Ilana by my side is my best gift.

"Creating opportunities for others makes me so happy. And I'm not done yet!"

King received well wishes from fans around the world and a smattering of famous faces in the comments of her Instagram post, including gold-medal winning Olympian Lindsey Vonn, who wrote that King is "always [her] hero," and actress Reese Witherspoon, who said King is "such an inspiration" to her.

Her 80th birthday closes out what was an eventful 2023 year for King, which saw her guest star on the "Masked Singer;" walk the halls of Congress as she was nominated for the legislature's highest civilian honor, the Congressional Gold Medal; and join with former First Lady Michelle Obama to celebrate 50 years of equal prize money at the US Open, where she also reunited with WTA stars past and present to celebrate a half-century of the women's tennis tour.

And her commitement to advancing women's sports remained as strong as ever: In August, she and longtime partner Kloss traveled to Australia to support the women's World Cup, celebrating with Spain as it won its first-ever crown, and that same month, announced a transformative investment in the Billie Jean King Cup, the team competition of women's tennis that now bears her name.