Serena Williams' huge secret as she and Venus Williams made history at 2017 Australian Open

Serena Williams was keeping a huge secret when she faced off with sister Venus Williams in the women's singles final at the 2017 Australian Open.

The American tennis stars had gone head-to-head in the final 14 years prior, at the 2003 AO, where Serena took home the win to huge acclaim.

But something was different when she stepped onto the court to face her older sister in 2017. Not that anyone else knew it at the time.

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Serena came out swinging, securing the first set 6-4 as sister Venus battled to beat her younger sister.

Unfortunately for Venus, she simply couldn't outplay Serena, who won the second set 6-4 and in doing so secured her 23rd Grand Slam title. Aged 35 at the time, she was oldest woman to win a major in the open era.

"I am trying to stay up here as long as I can. I missed it last year," Williams said after her win, dashing any rumours that she planned to retire.

She also paid tribute to older sister Venus, without whom Serena said she never would have achieved 23 Grand Slam titles.

"Thank you Venus for inspiring me to be the best player that I could be and inspire me to work hard every time you won this week I felt like I got a win too," she said.

There were no hard feelings from Venus, who seemed just as happy to see her little sister make history at the AO that year.

"Congratulations Serena on number 23. I have been right there with you, some of them I lost right there against you," she laughed.

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The match made history in several ways, marking 35-year-old Serena's return to the world No. 1 singles spot and her 23rd major singles title.

It would also be her last major singles title, as well as the last time she and Venus would face off in a major tournament final.

And Serena did it all while about two months pregnant with her and husband Alexis Ohanian's first child, Alexis Olympia Ohanian Jr.

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The tennis champion announced she was expecting in April 2017, just a few months after her AO win, revealing to the world that she dominated at the tournament while pregnant.

Serena welcomed her first daughter in September 2017, then announced her second pregnancy at the Met Gala in 2023. Her second daughter, Adira River Ohanian, was born in August that year.

She retired from tennis in 2022, at which point she and Venus had faced off in 31 professional tournaments and Serena had won 19 to Venus' 12.

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