The tennis star, who will take part in the Eastbourne Aegon International this week, received medical treatment after suffering a pulmonary embolism and hematoma in March.
In an interview with BBC News marking her return to tennis, she said: "I'm so excited to be back. I was on my death bed at one point - quite literally."
Williams, 29, went on to say that she "didn't appreciate" the seriousness of the condition at first, but soon realised that if she had delayed seeing a doctor it could have meant the end of her career.
"They told me I had several blood clots in both lungs. A lot of people die from that," she said. "It got to the stage where it felt like I could hardly breathe. Some days I didn't get out of bed at all. I just laid on a couch thinking, 'Why has this happened to me?'"
The blood clots are thought to have developed after Williams cut her foot on glass while leaving a restaurant with her nephew and hitting partner and as a result, had restricted mobility for several months.
Williams, who has not played competitively since her Wimbledon win, added that the affliction has given her a "whole new perspective" on her life and career.
She was recently reported to have begun work on a series of rap tracks with Minnesota Viking Bryant McKinnie's record label and producer DJ Clue.
