Writing for Vogue, the American 23-time Grand Slam singles champion said she is moving towards "other things that are important to me", adding she does not like the word "retirement".
In an accompanying Instagram post, she wrote that "the countdown has begun".
"I'm gonna relish these next few weeks," the 40-year-old said.
Williams made her singles return at Wimbledon in June after a lengthy injury lay-off that had led to speculation about her retirement.
She says she will play at the US Open, where she has won six of her major singles titles, later in August.
Williams has won more major singles title than any female player in the Open era and the second most of all time, behind Margaret Court's 24.
"Unfortunately I wasn't ready to win Wimbledon this year. And I don't know if I will be ready to win New York. But I'm going to try," she wrote.
"I know there's a fan fantasy that I might have tied Margaret that day in London, then maybe beat her record in New York, and then at the trophy ceremony say, "See ya!"
"I get that. It's a good fantasy. But I'm not looking for some ceremonial, final on-court moment.
"I'm terrible at goodbyes, the world's worst. But please know that I am more grateful for you than I can ever express in words."
On Monday, she had her first singles win in 14 months, beating Spain's Nuria Parrizas Diaz to reach the second round of the National Bank Open in Toronto.
'It is the hardest thing I could ever imagine'
In the lengthy essay, Williams says she has been "reluctant to admit to myself or anyone else that I have to move on from playing tennis".
"Alexis, my husband, and I have hardly talked about it; it's like a taboo topic," she wrote.
"I can't even have this conversation with my mom and dad. It's like it's not real until you say it out loud. It comes up, I get an uncomfortable lump in my throat, and I start to cry.
"I know that a lot of people are excited about and look forward to retiring, and I really wish I felt that way. "
In singles, Williams has won the Wimbledon and Australian Open seven times, along with six US Open trophies and three French Open titles.
Her first major title was the 1999 US Open, when she was 17.
She won her last Slam title at the 2017 Australian Open when she was eight weeks' pregnant with daughter Olympia.
She says there is "no happiness" in the possibility of no longer playing tennis.
"I know it's not the usual thing to say, but I feel a great deal of pain. It's the hardest thing that I could ever imagine," she wrote.
"I hate it. I hate that I have to be at this crossroads. I keep saying to myself, I wish it could be easy for me, but it's not.
"I'm torn: I don't want it to be over, but at the same time I'm ready for what's next."
Williams nearly died in childbirth with Olympia but returned to the sport, reaching four major finals after coming back from maternity leave. She lost them all, leaving her, tantalisingly, one short of Court's record.
She discusses having more children in her essay, stating: "I definitely don't want to be pregnant again as an athlete.
"I need to be two feet into tennis or two feet out."
'I did it - and so can you'
Williams is one of sport's most iconic stars, attracting full stadiums and devoted fans throughout her career.
She and older sister Venus have been widely regarded as changing the face of tennis and inspiring future players, as well as pushing for gender equality.
Williams describes the "essence of being Serena" as "expecting the best from myself and proving people wrong".
"There were so many matches I won because something made me angry or someone counted me out. That drove me," she said.
"My sister Venus once said that when someone out there says you can't do something, it is because they can't do it. But I did do it. And so can you."
Williams also won 14 Grand Slam doubles titles alongside Venus, two mixed doubles Slam titles and four Olympic gold medals in singles and doubles.
As well as her many highs she had several near-misses when it came to adding to her major tally, including emotional exits at the Australian Open and Wimbledon in 2021.
She also had some controversial moments at the US Open, including the 2018 final in New York where she accused umpire Carlos Ramos of being a thief.
Ranked by Forbes as the highest-earning female athlete of all time, Williams has moved into business in recent years, starting a venture capital firm.
She is also part of a group that funded a new Los Angeles-based women's team in the National Women's Soccer League season.
"I don't particularly like to think about my legacy. I get asked about it a lot, and I never know exactly what to say," Williams added.
"But I'd like to think that thanks to opportunities afforded to me, women athletes feel that they can be themselves on the court.
"They can play with aggression and pump their fists. They can be strong yet beautiful. They can wear what they want and say what they want and kick butt and be proud of it all.
"I don't know how I'm going to be able to look at this magazine when it comes out, knowing that this is it, the end of a story that started in Compton, California, with a little Black girl who just wanted to play tennis."