splashnews.com (top), Storms Media Group (bottom)

Looks like things are heating up between John Mayer and Jessica Simpson.

The 26-year-old songbird is accompanying her musician boyfriend for part of his tour in Florida (his latest album, Continuum, was released in September).

She arrived in Miami early on Monday to meet up with Mayer, 29, at the Four Seasons Hotel, where he's staying while he's in town working on some music with Sony.

"Jessica flew in very early after a red-eye flight on Monday morning, and spent the day with her boyfriend," a source tells Us. "They went to the gym and the spa before going out for the evening.”

Later that night, the couple got cozy over a romantic sushi dinner at the Shore Club’s Nobu in South beach. They arrived in a black Town Car and entered the restaurant holding hands, with Simpson looking pretty and feminine in a halter dress and heels.

Behind a heavily-guarded table for two, the singers shared a leisurely, two-and-a-half hour dinner of raw fish, spicy shrimp and salad.

“They looked into each other’s eyes and talked the whole time,” says a witness. “They were laughing and really into the other’s conversation. They both looked extremely happy.”

The pair left hand-in-hand just after 11:30 p.m., and drove back to their hotel where, a source tells Us, they spent the night together in the Four Seasons' Presidential Suite.

According to the source, on Tuesday afternoon, the couple will hop on Mayer’s tour bus and travel together to Jacksonville, Florida, where he’s playing Thursday night, and return to the following day for Mayer’s concert in Miami.

Talk to Us: Whose music do you prefer -- Mayer's or Simpson's?

If We Ruled Seattle Grace