The confirmation came hours after the 35-year-old athlete posted a picture of herself in a yellow one-piece swimsuit on the photo-sharing site Snapchat with the caption: "20 weeks."
Williams, who will return to the No. 1 ranking in the world next week, deleted the photo shortly after it appeared. She waited more than six hours to make any public comment, stoking furious debate among tennis fans about whether or not she was joking.
"I'm happy to confirm Serena is expecting a baby this fall," Los Angeles-based Bush Novak said.
If Williams is indeed 20 weeks into her term, that would mean she was approximately two months pregnant when she captured her record 23rd Grand Slam tournament title at the Australian Open in January.
Her victory over older sister Venus Williams broke a tie with Steffi Graf for the most Grand Slam singles championships in the sport's modern era. Serena Williams has not played since.
Fans and celebrities reacted with a mixture of awe and glee to Williams' Snapchat post.
"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," wrote Twitter user @noelleharmony.
"So Serena cheated," Trevor Noah, the host of "The Daily Show" on Comedy Central, wrote on his Twitter account. "Because that's technically a doubles match."
Williams, the world's highest paid female athlete, announced her engagement to Reddit co-founder Alexis Ohanian last December.
The Women's Tennis Association initially congratulated Williams on Twitter before deleting its post.
Tennis' U.S. governing body, the United States Tennis Association, wrote on Facebook, "Join us in congratulating the GOAT and her fiancé Alexis Ohanian on this incredible news!" referring to Williams with the acronym for "Greatest of All Time."
The next Grand Slam tournament this year is the French Open, which starts in May. If Williams is 20 weeks pregnant and gives birth after a full term, she would have her child in early September, during the fourth Grand Slam, the U.S. Open in New York.
Other women have left the tour to have children and returned at a high level, though none has done so at Williams' age. The Belgian player Kim Clijsters retired and had a child before coming back at age 26 and winning three Grand Slam titles as a mother.
Australians Evonne Goolagong and Margaret Court also won Grand Slam titles after having children.
Former No. 1 Victoria Azarenka of Belarus, 27, missed most of the 2016 season to have a baby and is expected to return to the tour this summer.
For Williams, who commands nearly $29 million in salary and sponsorship earnings, becoming pregnant makes her all the more attractive to corporate sponsors, industry executives said.
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.
Williams, who has played professionally for two decades, first gained the No. 1 ranking in 2002 and has held that spot for more than six years in total during her career.
Sister Venus, 36, has won seven Grand Slam singles titles.
(Reporting by Frank Pingue in Toronto; Additional reporting by Steve Keating in Toronto, Jonathan Allen in New York and Dan Whitcomb in Los Angeles; Writing by Steve Gorman in Los Angeles; Editing by Grant McCool and Richard Chang)
