WordPress custom post type – permalink issue

I am having the following problem. I have created a custom post type called services. When I create a new service, lets call it xpto, the permalink looks something like this domain/services/xpto. And I would like the permalink to be something like this domain/xpto.

My Settings->Permalinks are set to Post name domain/sample-post, and in the function I used to create the post in functions.php, I have tried several options in the rewrite argument, but to no success.

Help please.