The 35-year-old will return to world number one on Monday despite her recent inaction but she is unlikely to play again this year if the pregnancy is confirmed. She holds 39 major titles, the most amongst active players.
Association, organizers of the US Open which Williams has won six times tweeted, using the acronym for "Greatest Of All Time".
But the story spread quickly across the internet and soon the congratulations were flooding in from all quarters, both for Williams and for her fiancé Alexis Ohanian, the online entrepreneur who founded the Reddit network.
April 19 A pregnant Serena Williams makes the tennis superstar even more attractive to corporate sponsors, allowing her to extend her reach into maternity wear and motherhood products, sponsorship industry executives said. Others have expressed their awe and disbelief that Williams won her seventh Australia Open while pregnant.
Australians Evonne Goolagong and Margaret Court also won Grand Slam titles after having children. The two got engaged back in December after dating for about 15-months. Victoria Azarenka, 27, had a son in December and is expected to be back on the tour in July. The next Grand Slam tournament is the French Open in late May and early June, Williams will presumably not compete in that tournament if she is pregnant.
Without Williams, who was next due to play in May's Madrid Open, there would be a massive vacuum and a near free for all when it comes to the battle for the sport's biggest prizes.