Williams, 35, wrote of her admiration for her mom, for her strength in the face of those "too ignorant to understand the power of a black woman."
In the letter, the 23-time grand slam champion -- the most successful player in the Open Era -- talks of the criticism she has had to endure for her muscular physique and writes of her baby daughter's "same strong, muscular, powerful, sensational arms and body."
The American opens the letter by calling her mom "one of the strongest women I know."
'We are women and proud'
"I was looking at my daughter (OMG, yes, I have a daughter) and she has my arms and legs!" Williams says in the letter first published on reddit,
the site co-founded by her fiance Alexis Ohanian.
"I don't know how I would react if she has to go through what I've gone through since I was a 15-year-old and even to this day.
"I've been called man because I appeared outwardly strong. It has been said that that I use drugs (No, I have always had far too much integrity to behave dishonestly in order to gain an advantage).
"It has been said I don't belong in Women's sports -- that I belong in Men's -- because I look stronger than many other women do. (No, I just work hard and I was born with this badass body and proud of it).
"But mom, I'm not sure how you did not go off on every single reporter, person, announcer and quite frankly, hater, who was too ignorant to understand the power of a black woman.
"I am proud we were able to show them what some women look like. We don't all look the same. We are curvy, strong, muscular, tall, small, just to name a few, and all the same: we are women and proud!"
In describing her mom as "classy," Williams thanks Price for being the role model she needed to "endure all the hardships that I now regard as challenges" and talks of inheriting her mom's fortitue.
The letter ends with Williams writing: "Promise me, Mom, that you will continue to help. I'm not sure if I am as meek and strong as you are yet. I hope to get there one day. I love you dearly."
Before the birth of her daughter, Williams told Vogue
she wanted to defend her Australian Open title in January 2018.
"It's the most outrageous plan," she told the fashion publication in August. "I just want to put that out there. That's, like, three months after I give birth. I'm not walking anything back, but I'm just saying it's pretty intense."