Katy Perry and John Mayer are both single again. 

The couple have reportedly split for the second time in less than a year. "It's sad," one pal says, but adds that a third go-round is possible. "It's not over until it's over. You have to see how things play out." Another source says, "She's leaving the window open. They have both been so focused on work."

The two singers began dating last summer, but broke up in August before reuniting a month later.