Katy Perry and John Mayer have called it quits for the second time, sources tell Us Weekly. The pop star and singer-songwriter started dating last summer then split briefly in August before reconciling the following month.

"It's sad," one friend told the magazine, though added, "It's not over until it's over. You have to see how things play out." What started as a casual relationship seemed to ramp up following their September reconciliation, including making visits to Perry's hometown and appearing together at events like this year's Grammys.

Video: Katy Perry Performs 'Teenage Dream' at the Kids' Inaugural Concert

Another friend pointed to both musicians' busy schedules, saying that Perry is "leaving the window open. They have been so focused on work." Perry is in the midst of recording her third studio LP, while Mayer is about to start touring again following major surgery to repair a granuloma in his throat.

In a chat with Rolling Stone this past January, Mayer said of his relationship with Perry: "I'm quite happy. I'm happy in all aspects of my life. I'm very happy in all aspects of my life."

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