The photo, captured by US media, shows Williams in a yellow swimsuit with what appears to be a baby bump.
The timing means that the 35-year-old American would have been pregnant when she won her record-setting 23rd Grand Slam title at the Australian Open in January -- where she beat her sister Venus in the final.
"The GOAT herself @serenawilliams took to Snapchat to announce she's 20 weeks pregnant! Congratulations Serena!" the US Tennis Association, organizers of the US Open, tweeted.
Fans, too, reacted with astonishment on social media.
"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," tweeted @noelleharmony.
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 poised to return to regain the top spot from Germany's Angelique Kerber on Monday.
In December, Williams announced her engagement to Reddit co-founder Alexis Ohanian, first sharing that news on the Reddit social media site.
While pregnancy may spell the end of Williams's season, it doesn't mean she won't be back on court.
