Two bros were playing a casual game of tennis. Then Serena Williams showed up.

It's a crisp evening as you and a friend play a casual game of tennis. You're getting the blood flowing, breathing some fresh air, and decompressing just a bit. Casual. And then -- holy crap is that Serena Williams?!?

Yes, that is Serena Williams. What's more, she says this story is one with a moral: You never know when the GOAT might crash your game of tennis.

Williams was out for a stroll in San Francisco's Mission District on Sunday night when she came across two chill bros playing some tennis at Dolores Park. She was wearing a pair of fuzzy slipper-like boots instead of her usual tennis shoes -- but that didn't stop her from jumping in the game.

The very awesome encounter -- from Williams first spying the game, to the two players' stunned reactions -- was documented on her Snapchat channel. Here's a look, via ForTheWin's Alysha Tsuji.

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"So the moral of the story is," Williams says, while a bit breathless after the game, "you never know when I could be coming to a tennis court near you."

But it probably helps that this particular tennis court happens to be located in San Francisco, where Reddit is headquartered. Reddit's co-founder is Alexis Ohanian, to whom Williams became engaged last December.

But still. We won't blame tennis players the world over for holding out just a little bit of hope now that Williams might crash their own game one day.

Perhaps just as remarkable in these mega-branded times, the encounter appears to be just a bit of spontaneous fun, not some native ad for a sneaky company. That might be the best part of all of this.