Serena Williams used a stand-in baby in advert everyone thought featured her daughter

Tennis superstar Serena Williams used a baby body double in her latest commercial for sports drink Gatorade.

Many thought the newlywed champion filmed it with her nearly three-month-old daughter Alexis Olympia, but it has been revealed it was another baby she was holding, AdAge reported.

A Gatorade spokesperson said in a statement to AdAge that Williams "intended to have Olympia make her screen debut as part of this film, her daughter was under the weather and unable to be on set, but the sentiment remains the same."

Williams said, while holding the baby, in the two-minute advertisement: "Baby girl, I won’t mind if you play tennis badly. I won’t mind if you choose to never pick up a racket. But I beg you, in this game of life, please keep playing no matter what."

Ms Williams was actually two months pregnant with the baby when she won the Australian Open earlier this year, finding out the test results just a week before she began the tournament. She did not drop a single set on her way to victory.

Williams and husband Reddit co-founder Alexis Ohanian have put baby Olympia in the spotlight already - she has her own Instagram account with over 142,000 followers.

Ohanian also posted a video to YouTube "beginning with an ultrasound and ending with footage of the baby leaving the hospital after six days," according to the news site.

The campaign, titled "Sisters in Sweat," aimed to get more girls involved in sports because the brand said girls drop out of sports at 1.5 times the rate of boys by the age of 14. Gatorade's parent company PepsiCo CEO Indra Nooyi tweeted that "we can all learn" from Williams' words in the spot.

Partnering with lifestlye website Refinery29 for research, Gatorade said: "One of the primary reasons girls drop out of sport is because they don't see a future in sport and believe their time is better spent preparing for their future in other ways" despite staying in sport leading to being "healthier, happier and more successful later in life."

Williams and Ohanian married on 16 November in a lavish ceremony in New Orleans, Louisiana.