Los Angeles - Serena Williams is expecting a baby and will not play again this year but aims to return to the women's tour in 2018.
The 35-year-old posted a picture of herself captioned "20 weeks" and her publicist Kelly Bush Novak made an official announcement.
"I'm happy to confirm Serena is expecting a baby this fall," the publicist said in a statement to AFP.
While Williams won't play again this year, "she looks forward to returning in 2018," Bush Novak said.
Williams' social media post showed the 35-year-old in a yellow swimsuit that revealed the baby bump.
But the post was quickly taken down, lending an air of mystery to its meaning.
"The GOAT herself @serenawilliams took to Snapchat to announce she's 20 weeks pregnant! Congratulations Serena!" the US Tennis Association, organisers of the US Open - which Williams has won six times - tweeted, using the acronym for "Greatest Of All Time."
The Women's Tennis Association deleted one congratulatory tweet because they did not have confirmation of but later said: "It's official! @SerenaWilliams and @AlexisOhanian are expecting a baby this fall."
In December, Williams announced her engagement to Reddit co-founder Alexis Ohanian, first sharing that news on the Reddit social media site.
If Williams is 20 weeks into her pregnancy, that means she was already expecting when she won her record-setting 23rd Grand Slam title at the Australian Open in January - where she beat her elder sister Venus in the final.
For fans and friends the announcement underscored Williams' superwoman status.
Retired ATP Tour player Andy Roddick cheered the news.
"There's gonna be a baby GOAT ..... We are so happy, and we know @serenawilliams will be a great mother," the fellow American wrote on his Twitter account.
Williams hasn't played since her Australian Open triumph, which saw her surpass Steffi Graf for the most Grand Slam singles titles in the Open Era.
Only Australian Margaret Court has won more, capturing 24. The last of those came in 1973 - the year after Court gave birth to a son.
Williams cited a knee injury in pulling out of tournaments in Indian Wells and Miami in March.
Her inactivity saw her drop to second in the world rankings. But she is set to regain the top spot from Germany's Angelique Kerber on Monday when the latest standings are published.
Williams wouldn't be the first top women's player to return to competition after having a baby, but she would be the oldest.
Belgium's Kim Clijsters came out of retirement at the age of 26 after having a child and won three Grand Slam titles.
Former world number one Victoria Azarenka of Belarus, 27, won the Indian Wells and Miami titles in March of last year before missing the rest of the season as she awaited the birth of her first child in December.
Since the birth of son Leo, Azarenka has returned to training and plans to return to competition at the WTA tournament in Stanford, California, in July.
But Williams has proved herself capable of returning to top form after long absences from competition.
She was sidelined for almost 12 months after her 2010 Wimbledon victory, returning from a potentially life threatening pulmonary embolism.
Over the past year Williams' schedule has included few tournaments outside the Grand Slams.
Her appearance at the Australian Open tuneup event in Auckland, New Zealand, marked her first tournament since a semi-final exit at Flushing Meadows last September.
Factfile on 23-time Grand Slam singles champion Serena Williams, who
announced on Wednesday she is expecting her first child this year:
World ranking: 2
Age: 35 (26/09/1981)
Career titles in total: 71
Career Grand Slam titles: 23 - Australian Open: 2003, 2005, 2007,
2009, 2010, 2015, 2017; French Open: 2002, 2013, 2015; Wimbledon: 2002,
2003, 2009, 2010, 2012, 2015, 2016; US Open: 1999, 2002, 2008, 2012,
Career prize money: $84 463 131
- Deposed by Angelique Kerber as world number one last year, Williams
regained the top ranking with her Australian Open victory in January,
although inactivity since then saw her drop back to No. 2.
- With her 23rd Grand Slam title she surpassed Germany's Steffi Graf for the most in the Open Era.