Serena Williams engages in thoughtful conversation with Selena Gomez: Watch

Serena Williams opened up about a personal subject surrounding her mental health and spoke about how he dealt with it.

In an Instagram clip shared by Selena Gomez on Monday, the pair dove into the topic of "mental fitness" while engaging in a thoughtful conversation for the former Disney star's mental health platform, Wondermind.

"Mental fitness for me is just really learning [ing] to shut down," Williams, 40, said. "And you know I did this years ago before even mental health was a topic among everyone's mind."

"It was more just like, alright, I'm shutting myself down today. Just subconsciously, it was something I've always done," she continued. "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."

"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," added Williams, before concluding, "And then when I'm turned off, I'm turned off."