VIDEO: Serena Williams' daughter Olympia help little sister workout

Tennis legend Serena Williams on Christmas Day shared an adorable video of herself already teaching her daughters, six-year-old Olympia Ohanian, and four-month-old Adira River, how to train together.

The 42-year-old tennis pro, who shares both daughters with husband Alexis Ohanian, posted an Instagram video of her daughter, Olympia helping her baby sister Adira "work out" on Christmas.

In the video, Williams instructs her eldest daughter, wearing a ruffled red dress, in French, as she lifts and lowers her baby sister, wearing holiday pyjamas.

Williams is heard telling her daughter, "Doucement, doucement... comme ça," which translates to "gently, gently ... like that", to emphasise the importance of gentle movements, akin to baby sit-ups.

Olympia's actions are a sweet and engaging experience for her sister.

Williams captioned the video: "We all work out in this family. @adiraohanian @olympiaohanian".

The 23-time grand slam champion disclosed last month in November that Olympia is "navigating" her role as Adira's elder sister.

"She loves it," Williams told Entertainment Tonight at the Council of Fashion Designers of America Awards. "Adira's like a tiny little baby, so Olympia just calls her her little sis."

Williams explained that while Olympia "prayed for a sister," she herself was a little "worried" ahead of Adira's arrival.

"I was like, 'OK, I don't know if I can like anyone as much as I love Olympia,'" Williams said. "I was really nervous about that. But I feel like it all worked out."

Following the birth of her second kid, Williams and Ohanian have released pictures of the newborn and revealed some of the more difficult aspects of being a mother.