The American, who has won 21 Grand Slam singles titles, will chase you down, and she will catch you.
It was a lesson learnt by a man who took the 34-year-old's mobile phone while she was in a restaurant.
"Yesterday at dinner the craziest thing happened to me," the world number one posted on her Facebook page, accompanied by a picture of herself as Superwoman.
Williams, who is taking the rest of the year off to recover from injuries, claimed the man "grabbed her phone and swiftly left".
She said: "Not thinking I reacted (hence the superwoman photo), I jumped up, weaved my way in and out of the cosy restaurant (leaping over a chair or two) and chased him down."
Williams said she was "too fast" for the man and was "upon him in a flash".
She added: "In the most menacing, yet calm, no-nonsense voice I could muster, I kindly asked him if he 'accidentally' took the wrong phone.
"He stumbled on his words, probably not expecting this to happen. Eventually he said: 'Gosh, you know what, I did! It was so confusing in there. I must have grabbed the wrong phone.'"
Williams, who won every Grand Slam singles title except the US Open this year, said she received a "standing ovation" from her fellow diners at the restaurant.
She described it as a "win for the ladies" and said she showed "every man in there I can stand up to bullies".
Here's a tip for anyone planning to grab Serena Williams's phone - don't bother.