Mapping a domain to subdirectory

I’ve been searching for solutions to this problem, but none of them works. Here is the problem I am struggling with:

I have a multisite and a separate wp installed at one host.
The multisite uses the domain name http://www.xxx.com while the separate wp is installed under http://www.xxx.com/abc
Now I’ve bought a new domain name http://www.abc.com and I want to map it to this separate wp.
Is there any way to do this?

I’ve tried to change the WordPress address in the setting tab, but when I typed in http://www.abc.com or http://www.xxx.com/abc it returns my multisite which is http://www.xxx.com

I am so frustrated and don’t know how to solve this. Please help, thanks!!