The top-ranked athlete performed "Diamonds Are a Girl's Best Friend" on Wednesday night at Arena in New York City. She danced and swayed her hips in a mini dress as she sang the song in falsetto and attempted high notes.
Williams told the crowd before she began: "I'm really nervous."
She was joined by fellow tennis player Jack Sock, "Orange Is the New Black" stars Laverne Cox and Uzo Aduba, "Glee" actor Darren Criss and comedian Kathy Griffin at the event for Delta Airlines.
Williams, a karaoke enthusiast, called the singing sport "my favorite thing."
"I do like 80s, 90s rock because it's good for my voice," she said before she performed. "The thing about karaoke is picking a song that works for your voice, that way if you sing like a cat - like I do - you can still sound pretty good."
Williams is seeded No. 1 as she seeks her third straight U.S. Open title. She said winning is "super important," but added that there's "no pressure."
"I feel good. I feel good. I'm looking forward to it," she said. "I am going to be relaxed and enjoy myself."
Sock, 21, said he and Williams will play together during next year's Hopman Cup tournament. He sang a lively rendition of R. Kelly's "Ignition."
Aduba, who recently won a Creative Emmy, didn't take the stage, but her co-star Cox drew in the small crowd at the private event with a rendition of Whitney's Houston "One Moment In Time."
"Please don't put this on YouTube," she said, laughing.
Cox joined Criss and Griffin onstage when they sang "Say My Name" by Destiny's Child.
"Everybody, Beyonce Knowles!" Criss yelled, introducing Griffin.