Los Angeles, California - Serena Williams got real with Selena Gomez in a conversation that touched on personal boundaries and making mental health a priority.

On Monday, the singer dropped a clip on Instagram of her poignant interview with the 40-year-old pro-tennis player for her website Wondermind.

During the candid chat, the stars got real about health, setting boundaries, and what it means to have "mental fitness."

"Mental fitness for me is just really learning to shut down," Williams shared with the 30-year-old actor.

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She explained that this was a habit she "subconsciously" developed throughout the years.

"And so now that I know that it's so important to just put yourself first, especially mentally, I always have shut down moments. I have serious boundaries and I don't let anyone cross those boundaries."

Williams also noted how much importance she puts on staying in a good space mentally – not just for herself, but also for her daughter Olympia, whom she shares with husband Alexis Ohanian.

"For me, it's so important to make sure, every day, I have a period of, like, it's so bad, because I really don't do anything for me, I'm terrible at that! And I've said it time and time again. I'm working on it. But more or less, at least prioritizing what I need to do," the athlete explained.

"And then when I'm turned off, I'm turned off."

Williams' honest interview follows her shocking retirement announcement earlier this month.

The era-defining tennis player penned an emotional essay for Vogue where she revealed that she will be "evolving away" from tennis to focus on expanding her family.