Williams and Alexis Ohanian, an American businessman and a co-founder of Reddit, announced their engagement in late December.
Arguably the greatest tennis player - male or female - based on achievements, quality of opponents, era and tenacity, Williams is only one Grand Slam win away from tying Australian Margaret Court's record 24.
Serena Williams has announced that she is pregnant, and while that is a wonderful development for her it will also have seismic impact on women's tennis. Amazingly, Williams won her seventh Australian Open title in January, which would have meant that she was around eight weeks pregnant.
The world number two posted a picture on social media site Snapchat, posing in a mirror with the message: "20 weeks", before deleting it.
The sports website further reported that Williams has not played any professional tennis following her Australian Open triumph.
The news broke Wednesday after Williams offered up a side profile shot of herself in a yellow swimsuit with a caption that read "20 weeks" in a Snapchat posting that quickly disappeared.
Researchers who study pregnancy in high-performing athletes said her training during pregnancy, the speed of her body's recovery after giving birth and her will power are the keys to Williams returning to top-level play.
Williams claimed her first Slam - the US Open - 18 years ago.
"Congratulations Serena!" the US Tennis Association, organizers of the US Open, tweeted, using the acronym for "Greatest Of All Time".
"I think the depth in women's tennis speaks for itself right now".
"I don't know (if Serena can win another) ... it will depend whether she still feels like doing it", Court said. The match was a notable one as Williams went up against her older sister, Venus.
Her baby is due in the autumn, the spokeswoman said.
The next Grand Slam tournament this year is the French Open, which starts in May.
It remains to be seen if both Williams and Azarenka can add to their haul of Slams upon their return, but if the former could it would be yet another incredible feat in her stunning career. She has won grand slam titles in her teens (one), 20s (12) and 30s (10, more than triple the previous mark). While she must not have known about her pregnancy at the time she must have been in the early stages of her pregnancy.