Sunday night, two men in San Francisco's Dolores Park found that out when out of nowhere the tennis legend walked up and asked if she could play them.
"Just having a stroll at night and I'm thinking about asking these guys if I could hit with them, just to see their reaction," Williams said in a video posted on her Snapchat page.
Williams' fiance, Alexis Ohanian, recorded it, capturing as she prepped for play. "Don't do this at home," she joked, referring to playing in UGG boots.
After winning the game, she said, ""The moral of the story is, you never know if I'm coming to a tennis court near you!"
Williams has been in San Francisco quite a bit, after getting engaged to Alexis Ohanian, the founder of Reddit. She posted video of the lead up to the match on Snapchat.