During Monday's kickoff of Timberlake's 36-date FutureSex/LoveShow tour, the singer said midway through the show, "Sometimes love isn't so easy," giving the crowd a clue in to his personal life.
Meanwhile, Diaz went solo to Tuesday's People's Choice Awards, where she won the trophy for favorite leading lady.
In their statement released Thursday, Diaz and Timberlake said: "We have, in fact, ended our romantic relationship, and have done so mutually and as friends, with continued love and respect for one another."
The pair were last seen out together Dec. 16 in New York City celebrating Timberlake's appearance on Saturday Night Live. The singer, 25, then flew home to Tennessee for the holidays while Diaz, 34, stayed in L.A. and then skied in Vail, Colo.
Diaz was also noticeably absent from the Jan. 3 premiere of Timberlake's movie Alpha Dog. Instead, he brought his mom as his date – and was spotted engaging in cozy conversation with Scarlett Johansson, who recently filmed a video for one of Timberlake's new songs, at a party afterwards.
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A source close to the couple said part of the problem was: "I don't think she's so much into the scene like he is. He's really young and he likes to go out. (He) hasn't been into her for awhile."
During his New York City visit with Diaz in December, Timberlake was spotted partying with two blonde models. At Christmas, he spent time in Memphis with ex-girlfriend Veronica Finn. (Finn, a Florida-based real estate agent, says he's just "an old friend.")
But don't expect fireworks, says Timberlake's pal. "They were laid-back together, so it's not going to be an ugly breakup. This is just them realizing it was time to move on."
And in an interview Friday with Ryan Seacrest on his KIIS-FM morning show in L.A., Timberlake said, "I'm great. I'm on tour and its awesome." When asked if being distracted by work was helpful, the singer responded: "Yeah, it's great."