Vogue magazine on August 9, Williams said she would be turning her attention from sports to her family and her venture capital firm, Serena Ventures.
Williams founded Serena Ventures in 2014 after learning during a JPMorgan Chase conference that less than 2% of all venture capital money goes to women.
"I realized then and there that someone who looked like me needed to start writing big checks. Sometimes like attracts like," she wrote.
According to Serena Ventures' website, 53% of the companies in its investment portfolio are founded by women, while 47% are black founders and 12% are Latino founders.
The firm has invested in more than 60 start-ups and announced in March that it had raised $111 million.
In February, Tonal announced that Williams would become a brand ambassador as part of its "Strength Made Me" campaign.