Watch: Serena Williams surprises two strangers by jumping into pick-up tennis match

Feb. 28 (UPI) -- Imagine playing tennis with your friend late at night in a park when a stranger asks if they can jump in.

Some people might shy away from the option, but two guys playing Sunday at Dolores Park in San Francisco are definitely glad they didn't.

The friends were wrapping up the last of three friendly matches at the park when a mysterious woman walked up and asked if she could jump in.

That woman was Serena Williams, the No. 1 ranked tennis player in the world.

"I'm going for a stroll at night and I'm thinking about asking these guys if I can hit, just to see their reaction," Williams said on her Snapchat. "I think they're in the middle of playing out points. I'm going to ask them if I can have the winner."

"I didn't want them to recognize me, but I think I heard them say my name already. Dangit," Williams said.

"Can I play the winner?"

"Yes, oh my god," one of the men said. "Oh my goodness...is this for real? Holy crap."

"Yes please," the men said.

"Don't do this at home," Williams said.

Williams entered the court with her dog, Chip, and swung a racket while sporting street clothes and boots.

She obviously won the match, saying she was still undefeated after its completion.

"The moral of the story is: you never know when I'm coming to a tennis court near you," Williams said.

Williams was walking through the park with her new fiancé Alexis Ohanian. Ohanian is the co-founder of Reddit.

Williams is a 23-time Grand Slam champion and is currently No. 1 in the WTA rankings. She is 8-1 this year with $2.7 million in prize money, according to the WTA.