Serena Williams is pregnant with her first baby.

The tennis star, 35, made the announcement on Wednesday by showing off her pregnant belly on Snapchat.

The athlete is engaged to Reddit co-founder Alexis Ohanian, 33.

Williams wore a yellow swimsuit and revealed she is 20 weeks along. The timing means she would have been around eight weeks pregnant when she won her record-setting 23rd Grand Slam title at the Australian Open in January - where she beat her sister Venus 6-4, 6-4 in 82 minutes.

The victory during her first trimester returned her to world No 1 and she also became the oldest grand slam winner in the open era.

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She's pregnant! Tennis star Serena Williams showed off her bump on Wednesday as she revealed is 20 weeks along with her fiance Alexis Ohanian's baby

The lucky guy: The beauty has been engaged to Alexis since late December 2016. The couple are pictured here in New Zealand in January; she shared this photo to Instagram last week

Serena's rep Kelly Bush Novak confirmed the news, writing in a statement: 'I'm happy to confirm Serena is expecting a baby this Fall.'

'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.

On Wednesday, Serena revealed her small baby bump, which she showed in profile as she held up her phone to a mirror. The suit had two gold buttons and a cut out on the side with narrow straps.

Soon after she posted the image to Snapchat - as well as to Instagram stories - she deleted it.

No hint of pregnancy here: Only a few weeks ago she showed off a smaller tummy

Later the Vogue cover girl shared Snapchat videos from her romantic vacation with her fiance.

In the clips, she shows off the view from a hilltop then a look at ancient ruins. She also includes a peek at her stuffed monkey named Max sitting on her hotel bed.

News of her pregnancy comes as a surprise as she has been showing off a flat tummy on social media in the past few weeks.

Already celebrating! Serena shared Snapchat videos from her vacation on Wednesday. In the clips she shows off ancient ruins (left) and her hotel bed on which sits her monkey Max

Alexis proposed to her in late December, around four weeks into her pregnancy.

He popped the question with a large diamond ring, which she showed off in January. They had been dating for only 15 months before he got down on one knee.

'It feels good,' she said of the engagement in late January.

'I really haven't thought about it too much, because I wasn't even really gonna think about it until after the tournament.

Loved up! Here the two are seen hugging in January soon after their engagement

'So, I just keep saying, 'February, I'll start looking at the bigger picture of my life.' But right now I'm just so focused that this is all I can think about.'

She also spoke glowingly of her husband-to-be: 'He's incredibly — a really nice person,' she said.

'I think he definitely treats everyone the same, which is really cool to see. Really, really nice to see.'

Williams has been romantically linked in the past to Grigor Dimitrov, Patrick Mouratoglou, Amar'e Stoudemire, Hosea Chanchez, Jackie Long, Common, Colin Farrell, and Drake, among others.

Williams is quite a catch for Alexis.

Champs: Serena was around eight weeks pregnant when she won the Australian Open in Melbourne on January 28 after defeating her sister Venus

She is one of the most recognized female athletes in the world: The Women's Tennis Association (WTA) has ranked her world No. 1 in singles on seven occasions, from 2002 to 2017.

The Florida resident also holds the most major titles in singles, doubles, and mixed doubles combined amongst active players.

The tennis ace is very wealth, too: according to Forbes, she is worth $150m.

Williams was born in Saginaw, Michigan, to Richard Williams and Oracene Price, and is the youngest of Price's five daughters: half-sisters Yetunde, Lyndrea and Isha Price, and full sister Venus.

She moved to LA's rough Compton neighborhood when she was still a toddler. When her career took off, the family moved to Florida.

A force: Williams, pictured during the Women's Singles Final against Venus at the Australian Open on January 28, is one of the most sucessful athletes in the world

Serena has made tennis the focus of her career, even though she has appeared on several TV shows (The Bernie Mac Show, ER and Law & Order: Special Victims Unit) and oversees a fashion line.

But now the star will have to take time off from the game, which she has rarely done since she started competing as a child.

The next Grand Slam tournament this year is the French Open, which starts in May. If Williams 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.