Casper Smart says Jennifer Lopez is his ''best friend''.

The back-up dancer has fallen head-over-heels for the 43-year-old singer and has told friends he is ''madly in love'' with her.

A friend of Casper, 25, said: ''He's madly in love with her. It's not just that she's his lover, she's also his best friend.''

Casper is so serious about Jennifer that he has been putting in a huge effort with her four-year-old twins Max and Emme, from her marriage to Marc Anthony, and loves being a surrogate dad to them.

The source added: ''He was going on about how he loves playing with them.''

It's not surprising Casper is making a big effort to get close to the kids, as Jennifer recently revealed being a mother is the most important thing in the world to her.

She said: ''Mommy. It's just the one thing you don't want to mess up. You can mess up in all other areas and get back up, but you don't want to mess up there.

''They're so precious to you, and they make everything better in my life. They make me feel better every single day. Definitely, it's the most important thing.''