Serena Williams has given birth to her second daughter, her husband announced on Tuesday, adding that both were "happy & healthy" after the difficult birth of the tennis star's first child left her fighting for her life.

"I'm grateful to report that our house is teaming with love: a happy & healthy newborn girl and happy & healthy mama," her husband, Reddit founder Alexis Ohanian, posted on Instagram alongside a photograph of himself with a beaming Williams hugging both her daughters.

Addressing his 41-year-old wife, he told her she had "given me another incomparable gift -- you're the GMOAT," a new maternal twist on the sporting accolade Greatest Of All Time.

In 2017 Williams, one of the greatest players in history, won the Australian Open -- one of her many Grand Slam titles -- while she was pregnant with the couple's first child, Alexis Olympia Ohanian Jr.

But months later she nearly died after she developed a blood clot in her lung after giving birth to Olympia via emergency C-section.

She spent six weeks bedridden after the pulmonary embolism, but battled back to make her return to competition just five months later in Fed Cup doubles alongside her sister, Venus.

Williams retired from tennis last year after becoming a superstar for a generation.

"I'm ready to be a mom and explore a different version of myself," she said after her final match last year.