SAN FRANCISCO - Tennis star Serena Williams should file a police report after she said someone tried to steal her cellphone at a restaurant, San Francisco police said Thursday.

Officer Grace Gatpandan said police would like to know if they are dealing with a serial cellphone thief.

Williams said on her Facebook page that she was having a Chinese dinner on Tuesday when she noticed a man lurking near her table. Her cellphone was on the chair next to her, and at some point the man grabbed it and left, she said.

Surveillance footage shows Williams leaving the restaurant and confronting the man on the sidewalk. Williams said she asked the man if he had accidentally taken the wrong phone, and he said he had before giving it back.

Danny Snyder, an employee at Mission Chinese Food, where Williams was dining, said the restaurant has never previously had anyone come in and try to steal a phone. He said Williams has eaten at the restaurant before.

Williams' Facebook post shows her in a 'Supergirl' costume.

"Always keep your things close," she wrote. "Fight for what's right. Stand for what you believe in! Be a superhero!"

Gatpandan had a slightly different message. She said police don't encourage people to go after cellphone thieves.

"The thing with cellphone snatchers is you don't know who you're dealing with," she said. "You don't know if they're a serial thief. You don't know if they're armed."