"Jake and Ruth were friendly and having a good time," an onlooker tells PEOPLE

By Melody Chiu
Updated January 14, 2015 05:30 PM
Credit: Walter McBride/WireImage

Ruth Wilson just might be having the best birthday ever.

Fresh off her big win at the Golden Globes on Sunday, The Affair actress – who turned 33 on Jan. 13 – took her talents to Broadway with costar Jake Gyllenhaal for the opening of their new show, Constellations.

Continuing their celebrations from Sunday night (the duo partied with the actor’s sister Maggie Gyllenhaal, who won best actress in a miniseries or TV movie, at various post-Globes parties), Wilson and Gyllenhaal, 34, had a large turnout for their big night.

Singing “Happy Birthday” to Wilson, Gyllenhaal and other guests – including 50 Cent, Jason Biggs, Derek Hough and Jerry Seinfeld – surprised the actress with a birthday cake.

“They all sat together in one area,” an onlooker tells PEOPLE about the pair’s families at the after-party, which was held at Urbo. “Jake and Ruth were friendly and having a good time. Everyone was celebrating her birthday and Golden Globe award.”

The duo – whose new play opened to rave reviews – have seemed cozy in the last few months, but a rep for the actor denies rumors of a romance.

Reporting by K.C. BAKER

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