I am aware at one time Directi was providing domain reseller accounts and that now they have the resellerclub.

I was checking most of the the billing softwares out there (awbs, clientexec, whmcs, etc) and they have support for the Directi API.

Is the Directi API and Resellerclub API, one and the same? If a certain billing software says it support Directi API, does that mean it also supports resellerclub's API?

should be.

just confirm with software vendor to make sure

I think Directi and Resellerclub's API same, and I chose clientexec thats graet billing software.

Yes, that should support. As they have just renamed their brand name. There's no other change.