Pippa Middelton attended the 2012 U.S. Open in Flushing Meadows, NY, on Tuesday, Sept. 4, but sources have come forward to reveal what may be the real reason for her overseas trip - to spend Labor Day weekend with hotelier Andre Balazs.

The 28-year-old was seen dining with the 55-year-old multi-millionaire on Shelter Island on Sunday, Sept. 2.

Balazs flew the Duchess of Cambridge's sister on a private plane to the island as his guest, according to several reports. Middleton stayed at one of his many Sunset Beach hotels and the pair were seen having dinner there shortly after her arrival.

The New York Post reported there was "definite chemistry between the pair."

"Pippa was wearing red jeans and a white top. She was dressed sexy, but not too sexy," a Hamptons resident told the newspaper, who also described their time together as "flirt central." "There was definitely chemistry. Andre was being very charming."

When Middleton was recognized by fellow diners, she posed for pictures while being "really nice and very friendly," the source added.

A second source said the two discussed business about the release of her upcoming book, "Celebrate: A Year of Festivities for Families and Friends."

"She shares his interest in hospitality, but everyone was speculating what a great romantic pairing this could be," the source said.

MTV Canada host Aliya Jasmine Sovani, who also happened to be dining at Sunset Beach that night, told Us Weekly that Middleton was recognized by fellow diners while waiting to use the ladies' room.

"She got really embarrassed when some people recognized her while she was in line," said Sovani. "She actually turned bright red!"

Middleton "didn't have security" and "was very low-key," Sovani added. She was also seen "walking around with bare feet, holding her wedges in her hand."

Middleton reportedly has a boyfriend in London, Alex Loudon, and Balazs reportedly dated actress Uma Thurman and E! late night talk show host Chelsea Handler.