The former World Number One tennis player revealed her pregnancy via an Instagram post on her 35th birthday, hailing the start of “precious beginnings”.
She posted a photograph of herself standing on a beach while beaming at the camera and cradling her growing baby bump.
Writing in the caption, the athlete said: “Eating birthday cake for two has always been my specialty.”
Friends and family rushed to congratulate the couple in the comments, including actor Lily Collins and Australian model Lara Worthington.
Gilkes, 42, also left a heart-eyed emoji under Sharapova’s post.
Sharapova has been engaged to Gilkes, a British businessman and friend of the Duke and Duchess of Cambridge, since December 2020.
The pair, who publicly confirmed their relationship in 2018, announced the engagement on both their Instagram accounts at the time.
The five-time Grand Slam champion said in her post: “I said yes from the first day we met. This was our little secret, wasn’t it @gilkesa?”
Gilkes wrote in the caption of his own post: “Thank you for making me a very, very happy boy. I look forward to a lifetime of loving you and learning from you @mariasharapova.”
The businessman has been linked to the royal family for more than a decade, having been a guest at the Cambridges’ wedding in 2011 and Princess Eugenie’s wedding to Jack Brooksbank in 2018.
Sharapova retired from tennis in February 2020, after 28 years in the sport. She is one of ten women, and the only Russian, to achieve the career Grand Slam.
Writing about her decision to retire in Vanity Fair, Sharapova said she would miss playing tennis every day.
“In giving my life to tennis, tennis gave me a life,” she wrote.
“I’ll miss my team, my coaches. I’ll miss the moments sitting with my father on the practice court bench.”