The birthday girl was treated to a party in the intimate wine cellar of the Forge, one of Timberlake's favorite eateries in the area, Monday night. His parents also attended.
"The group entered the restaurant around 9 p.m. through the front door and walked directly to the secluded wine cellar," an onlooker tells PEOPLE. "It is one of the prettiest places to have a party you can imagine."
Biel was surprised by an Italian wedding cake made specially by the chef as Timberlake serenaded her with a few lines of "Happy Birthday."
The couple looked "happy and comfortable" throughout the evening, the source says.
Timberlake, 33, was in town for a performance. His 20/20 Experience World Tour heads overseas later this month for stops in Europe and Asia.
Thanks for the birthday love! You guys know how to make a girl feel good. Or woman, I guess... Or close to middle aged woman? Ah, forget it.— Jessica Biel (@JessicaBiel) March 4, 2014
– Michele Corriston & Linda Marx