The Muse frontman organised a secret celebration for his fiancee's birthday at a friend's apartment in New York City last Saturday night (20.04.13), a day after she turned 34, where guests included Cameron Diaz, Judd Apatow and Leslie Mann.
A source told UsMagazine.com: ''Kate was totally surprised. The party started outside, but it was freezing so people moved inside and danced.
''Kate loves to dance and she was just really happy talking to everyone. She looked beautiful and beaming in a tight metallic dress.''
The couple, who have a 21-month-old son Bingham together, spent Kate's actual birthday in New Jersey, where Muse were performing at the IZOD Center in East Rutherford.
An insider said: ''They're very supportive of each other's careers. They're so happy together.''
Kate, who also has a nine-year-old son Ryder from her previous marriage to Chris Robinson, claimed she was looking forward to a low-key celebration for her birthday this year.
She previously said: ''I just want to hang with my family.''