Serena Williams back on the fitness train, tries to get her "ab muscles working again" under the watchful eye of daughter Olympia

Serena Williams is back working on her fitness again, this time under the watchful eye of her daughter Olympia.

The 23-time Grand Slam champion retired from tennis last year, playing her final tournament at the US Open in front of her home fans. Williams reached the third round in Flushing Meadows before falling to eventual quarterfinalist Ajla Tomljanovic in a tight three-setter.

Since then, the former World No. 1 has embraced life on the sidelines. Most recently, she welcomed her second daughter, Adira River Ohanian, in August with her husband Alexis Ohanian.

On Saturday, Serena Williams updated fans that she has resumed working on her fitness once again following her maternity break, especially focusing on her ab muscles.

Keeping a close eye on her training was her five-year-old daughter Olympia, who sat down in front of the American at the gym and watched her workout unfold.

"Olympia watching me try to get my ab muscles working again. Here comes fitness!" Williams captioned the post.

A glimpse at Serena Williams' home workouts

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Speaking to Vogue last year, Serena Williams had revealed that she was a big fan of home workouts. The American had given a detailed look into her training regimen, which involved a lot of HIIT (High Intensity Interval Training) workouts, stretching and cardio.

The 23-time Grand Slam champion's favorite type of cardio is running, followed by the elliptical machine.

"I don't necessarily want a trainer in my room or in the gym with me, so just having the Tonal trainers is a relief. It's just something different from what I've been doing for the past 20-plus years, and it's refreshing. But I definitely listen to my body. My favorite type of cardio is running, but my body has to be fit to be able to run or else my knees will hurt," Serena Williams said.

"After playing tennis for so many years, you just wear and tear down your knees, so the elliptical has been growing on me, too. I’m basically trying to do cardio every single day–even if it's, like walking a few miles, it still counts," she added.

Williams also spoke about her diet, emphasizing that she has been an on-and-off vegan for a while. For six days a week, the former World No. 1 eats vegan, but allows herself to eat other food she might crave on the seventh day.

"I've been an on-and-off vegan for a while. I was vegan for about six years and it was great. Now, I’m trying it out again because I really want to be healthy. And to be healthy, I need to eat healthy," Serena Williams said.

"When I don't eat healthy, I just feel terrible–to be honest, I feel like I almost have pain. So now, I eat vegan about six days out of the week. And then if I want, like, some eggs or something, I do that on the seventh day," she added.