How to set index page as parent of a structure
If I understand your question, you want to combine a Single and a Structure into the same family of URL's, something like this:

/services - (Single)

/services/service-01 - (Structure Entry)

/services/services-01/example-sub-service-01 - (Structure Entry)

/services/services-01/example-sub-service-02 - (Structure Entry)

/services/service-02 - (Structure Entry)

/services/service-03 - (Structure Entry)



As you rightly said, you would create a Single for your Services homepage and set the URI to services.

You would then create a Structure called "Servcies", and set the URI to:

{parent.uri ? parent.uri: '/services'}/{slug}

This nice bit of Twig checks if parent.uri is defined, and outputs it if it is. Otherwise (if the entry is top level in the structure, it just outputs /services/{slug}