"They already miss each other," says a source close to Mayer.
Adds another source, also close to the singer: "It's very real. They've been seeing each other for a few weeks now."
Aniston, 39, and Mayer, 30, were first seen together publicly last Friday when they had an intimate lunch at Michael's Genuine Food & Drink in the Miami Design District. Later in the day, the two were seen at Casa Tua on South Beach.
Aniston remained in Florida to shoot Marley & Me with Owen Wilson, hanging around a hotel with her gal pals. Mayer went home to New York, hitting the club Butter with buddy Josh Groban on Monday, then joining a friend for a meal at Il Mulino on Tuesday.