The tennis superstar, 36, revealed she began having difficulty breathing the day after her daughter, Alexis Olympia, was born via emergency C-section due to the baby's alarmingly low heart rate.
Alexis Olympia was fine after her birth in early September, but Williams tells Vogue in a new feature that "everything went bad" for her personally.
Williams' breathing issues were the result of multiple small blood clots in her lungs, and her coughing led to the laceration from the C-section to open up again. Doctors then discovered a hematoma — a collection of blood — was present in her abdomen during the subsequent surgery. Williams ultimately had to get a filter placed in a major vein to stop blood clots from going to her lungs, according to the magazine.
She was not able to get out of bed for six weeks after she finally left the hospital a week after giving birth.
Williams says the experience has taken an emotional toll on her as well.
"Sometimes I get really down and feel like, Man, I can't do this," she told Vogue. "It's that same negative attitude I have on the court sometimes. I guess that's just who I am. No one talks about the low moments-the pressure you feel, the incredible letdown every time you hear the baby cry."
She adds she's broken down frequently since becoming a mother.
"I'll get angry about the crying, then sad about being angry, and then guilty, like, Why do I feel so sad when I have a beautiful baby? The emotions are insane," she said.
But having a child has overall been a dream come true for Williams, who married Reddit co-founder Alex Ohanian in November and they welcomed their baby.
"Now that I'm 36 and I look at my baby, I remember that this was also one of my goals when I was little, before tennis took over, when I was still kind of a normal girl who played with dolls," she told the magazine.
Williams and her baby appear on the cover of Vogue's new February issue.
The tennis star, who has won a record 23 Grand Slam titles in her illustrious career, revealed last week that she will not be participating in the Australian Open later this month. She won the tournament last year while pregnant.
Williams played in an exhibition match late last month, which she lost to Jelena Ostapenko. That marked her first match since giving birth.