Serena Williams, a 23-time Grand Slam champion and iconic tennis superstar for a generation, said Tuesday that "the countdown has begun" to her retirement from the sport.

The 40-year-old American wrote a cover essay for Vogue magazine and an Instagram post in which she set the stage for "evolving away from tennis."

"There comes a time in life when we have to decide to move in a different direction," she said on Instagram. "That time is always hard when you love something so much. My goodness do I enjoy tennis.

"But now, the countdown has begun. I have to focus on being a mom, my spiritual goals and finally discovering a different, but just (as) exciting Serena. I'm gonna relish these next few weeks."

After suffering a torn right hamstring at Wimbledon last year, Williams was sidelined until playing doubles at Eastbourne in June and made a return at Wimbledon as a wild card, losing in the first round.

On Monday, Williams competed in her first hardcourt match in 18 months, defeating Spain's Nuria Parrizas Diaz to reach the second round of the WTA hardcourt event in Toronto, a tuneup for the US Open.

It was her first singles victory since the 2021 French Open, 14 months ago.

Williams is set to play next week at Cincinnati and compete at the US Open, where she won her first Grand Slam title in 1999 at age 17.

"I have never liked the word retirement. It doesn't feel like a modern word to me," Williams wrote in Vogue. "I've been thinking of this as a transition, but I want to be sensitive about how I use that word, which means something very specific and important to a community of people.

"Maybe the best word to describe what I'm up to is evolution. I'm here to tell you that I'm evolving away from tennis, toward other things that are important to me.

"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."

Williams won the most recent of her 23 Slams at the 2017 Australian Open when she was already pregnant with daughter Olympia.

She remains one shy of the all-time record of 24 Grand Slam singles titles won by Margaret Court, having dropped her past four Grand Slam finals at Wimbledon and the US Open in 2018 and 2019.

"I'm not looking for some ceremonial, final on-court moment," Williams wrote in Vogue. "I'm terrible at goodbyes, the world's worst."

Williams has won seven Wimbledon and Australian Open titles, six US Opens and three French Open crowns as well as the 2012 London Olympic singles gold medal and three more golds in doubles.

Fellow US tennis legend John McEnroe compared Williams to all-time NFL and NBA sports heroes.

"She's like an icon. Her place in American society has just gone to a place where she deserves it after everything she has accomplished," McEnroe told USA Today.

"She's sort of in that level where Michael Jordan, LeBron James and Tom Brady are. She's like one of the all-time greatest athletes in the history of any sport -- male or female."

She completed two "Serena Slam" sweeps of four consecutive majors, runs that began at the 2002 French Open and 2014 US Open and ended when she was upset in the 2015 US Open semi-finals by Italy's Roberta Vinci while going for a calendar-year Slam.

She has evolved into business interests but also wants to have another child.

"A few years ago I quietly started Serena Ventures, a venture capital firm. Soon after that, I started a family. I want to grow that family," she said.

"But I've been reluctant to admit to myself or anyone else that I have to move on from playing tennis."

Williams gave birth to Olympia in 2017 but was bedridden for six weeks after suffering a pulmonary embolism and said she doesn't want to be playing and pregnant again.

"I definitely don't want to be pregnant again as an athlete. I need to be two feet into tennis or two feet out," Williams said.

That's a line call only Williams can make.

"I'm turning 41 this month, and something's got to give," Williams said.

Williams said she feels like part of her legacy is that women athletes can feel freer to be themselves while competing with gestures and clothes and aggression.

"Over the years, I hope that people come to think of me as symbolizing something bigger than tennis," she said. (AFP)