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Williams instead channeled any nervous energy into singing at a karaoke night sponsored by Delta Air Lines, now in its second year. Before the show, she said she was more worried about her song choice than her tennis game.
"I'm going to make a game time decision. I always get so nervous every year like, `I don't know what to do, I don't know what to do,'" she said.
Williams, 33, ended up choosing "Under the Sea" from The Little Mermaid, complete with dance moves and Caribbean accent.
"We just kind of wanted to celebrate just kicking off (the) U.S. Open and having fun and relaxing and enjoying it," said Williams.
Kei Nishikori, who is seeded fourth for the U.S. Open, also was there.
After having to withdraw from the Cincinnati Masters because of fatigue and a hip injury, Nishikori said he's now doing well.
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"I'm feeling pretty good. After (Montreal) I took one week off," he said. "I couldn't play Cincinnati, but otherwise I'm feeling pretty good and my tennis is, you know, getting better and feeling very solid."
Other celebrities in attendance included Laverne Cox, Uzo Aduba and Jason Biggs of Netflix's Orange is the New Black, singer Estelle and actor Alan Cumming.