Pregnant pause - Serena is expecting first baby this fallSports | Apr 21, 2017
The 35-year-old posted a picture of herself on Snapchat captioned "20 weeks" before her publicist Kelly Bush Novak made an official announcement.
"I'm happy to confirm Serena is expecting a baby this fall," the publicist said. While Williams won't play again this year, "she looks forward to returning in 2018," she added.
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.
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.
Fans and celebrities reacted with a mixture of awe and glee to the news.
"Serena Williams was pregnant when she won her 23rd Grand Slam tournament at the Australian Open. And did not drop one set. Let that sink in," one fan tweeted.
"So Serena cheated," Trevor Noah, the host of The Daily Show on Comedy Central, wrote on Twitter. "Because that's technically a doubles match."
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 last month.
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 No 1 Victoria Azarenka of Belarus, 27, plans to return to competition at the WTA tournament in Stanford, California, in July, after giving birth to son Leo last December.