35-year-old Sharapova broke the news on Tuesday ... sharing a pic of her in a bikini top and sweats -- showing off her growing baby bump.

"Precious beginnings!!! 🤍👼🏻," Maria said on Instagram. "Eating birthday cake for two has always been my specialty ☺️🐣🎂"

The 5-time Grand Slam champ is welcoming the baby with her fiancé, businessman Alexander Gilkes. The couple has been together since 2018 ... and got engaged back in 2020.

Sharapova's celebrity friends shared their excitement for the couple ... including sports broadcaster Erin Andrews, Lily Collins and more.

Sharapova became the #1-ranked tennis player in the world back in 2005 ... racking up more than 600 singles wins in her career. She announced her retirement in 2020.

This is Sharapova's first child. She was previously linked to NBAer Sasha Vujačić and tennis player Grigor Dimitrov.