The 35-year-old posted a picture of herself in a yellow swimsuit with a prominent bump - she captioned the image "20 weeks".
The younger Williams sister is just one Grand Slam short of Margaret Court, who collected many of hers in an era when the difficulties of global travel often diluted the fields.
Williams claimed her first Slam - the US Open - 18 years ago. She later returned after the birth of her daughter and subsequently won at the Flushing Meadows, which was only the third tournament into her comeback.
With the Serena Williams pregnancy news on the rise, the tennis champ is set to return to the court in 2018. Any return would not be until well past her 36th birthday, which is bound to raise the question about whether a truly great career is at an end.
Serena Williams is the oldest woman to win a Grand Slam singles title and still dominates the tour at an age when most players have retired.
Serena Williams has not played a tournament since that victory. Serena Williams has not played since then, citing a knee injury. "I will keep moving forward and continue to be positive".
Williams, a 23-time major champion, was last seen participating in the Australian Open in January which she ultimately won.
Bill O'Reilly quits Fox News
O'Reilly - who has denied the claims - has been off the air since last week, when he announced he would be taking a vacation. While O'Reilly denied wrongdoing, dozens of his show's advertisers fled within days even as O'Reilly's viewership increased.
Apple to Dismantle Iconic Glass Cube at Fifth Avenue Store
Apple is expanding its flagship location from 32,000 square feet to 77,000 square feet, as The Real Deal originally reported. The $2 million job requires a site safety manager with the removal to be done by Englewood, NJ-based Waldorf Exteriors.