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On Monday night, Serena Williams played Francesca Schiavone in the first round of the U.S. Open in Queens, but she's been in the city for a few days beforehand to prepare.

It wasn't all work for the tennis star however.

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According to an onlooker, Williams, 31, who won the tournament for the fourth time last year, stopped into Dos Caminos in Manhattan last Thursday with a group of pals, including her agent and her assistant.

The group arrived late and didn't order food. Proof that Williams is preparing for the Open? While her friends enjoyed a round of the Mexican restaurant's margaritas, Williams only sipped water.