Imagine spending a lazy Sunday playing tennis with your friend and getting a surprise visit from the sport's GOAT.

That's exactly what happened in San Francisco last night, when Serena Williams took some time out of her evening stroll in the city's Mission Dolores Park with fiancé Alexis Ohanion to pull up on a couple of random dudes and ask if she could hit balls with them on the court.

Their reaction upon learning it was Serena herself on the other side of the fence?

Everything from "holy crap" to "oh, my God" and "is this for real?"

"I don't have on my Nikes," Williams said, pointing to her boots, "but these will work."

And just like that, Serena joined the fellas on the court, easily giving these dudes a story of a lifetime — something that they'll certainly be telling their grandchildren about someday.

Best of all, Ohanion captured the priceless moment on Snapchat for the world to see.

Watch the magic below.

And Serena ended it best, saying, "The moral of the story is you never know when I'll be coming to a tennis court near you."

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