Kate Middleton is the toughest woman for Serena Williams.

Williams and Meghan Markle have been friends for years. However, the tennis player considers the Duchess of Sussex's sister-in-law to be the most badass woman for her.

In an interview with InStyle for their first-ever Badass Women issue, the athlete was asked which celebrities belong to this category. Williams mentioned two singers, but wrapped up her list with the Duchess of Cambridge at the no. 1 spot.

"Well, obviously Beyoncé. That goes unsaid. Rihanna goes unsaid. Kate Middleton!" Williams said.

According to the celebrity mom, she could not imagine having a baby and then putting on makeup and go out to greet the public right after. Middleton did this following Prince Louis' delivery, so the royal mom tops Williams' list.

"No. And she was … standing!" she said, laughing. "Me too! So she's officially No. 1."

Back in April, Middleton only spent 12 hours in the hospital when she gave birth to her youngest son. Just seven hours from her delivery, the duchess reportedly stepped out for her newborn's public debut. According to reports, the Duchess of Cambridge always leaves the hospital immediately after she gives birth to avoid causing frenzy outside the facility where media and photographers camp. She knows that it would be disruptive to other patients, so Middleton makes speedy hospital exits.

However, unlike Williams who was impressed with how Middleton did it, another group of moms slammed the duchess for what stepping out looking pristine just hours after delivery. According to them, Prince William's wife was not inspiring.

"You know what I find troubling is all those young women saying 'Oh, it's inspiring," Meshel Laurie said on Australian TV programme "The Project." "That's not inspiring!"

"You know what would be inspiring? If when they came in her room and said 'Alright mate, up you get, hair and makeup, let's get out there' she said 'Don't be any idiot, get out! I've just had a baby!" Laurie continued.

Prince William and Middleton have already scheduled Prince Louis' christening. The couple will use the lavish Lily Font at the ceremony.

Kate Middleton
Kate Middleton is the toughest woman for Serena Williams. Pictured: Middleton during a tour of a traditional German market in the Central Square on July 20, 2017 in Heidelberg, Germany. Getty Images/Chris Jackson