Serena Williams' adorable 5-year-old daughter occupied herself by playing with a cat toy -- also referred to as a tampon -- in a TikTok video posted by the newly retired tennis legend.

The clip shows Olympia unwrapping a tampon and pushing it out of the applicator in the bathroom.

Williams asked Olympia what she was playing with ... and she responded, "It's a cat toy for Karma."

Serena was clearly amused, at first -- holding back laughter and making faces at the camera -- that is until Olympia continued to unwrap more "toys."

Side note: a box of tampons ain't cheap.

"Okay, no," Williams said, "Let’s not play with too many of those" ... but Olympia vowed to open only one more.

The 41-year-old thought the whole thing was dope, though -- captioning the video ... "Who am I to steal her joy, it’s a Cat Toy! 🗣Don’t @ me #OlympiasWorld."

Her fans thought it was hysterical, as well ... with one person commenting, "😂😂 At least it’s a cat toy. My son used them on a school project as clouds 😂😂😂."

Another fan gave Olympia an "A for creativity" ... while someone else said, "you bout to have to go to the store!" to restock on tampons.