Redirect a subdomain to the main domain

[snoopy74]

Again hello :o)

Creating a subdomain for an account, i have to specifiy a special directory.
Now one of my customers wants to receive all requests to a subdomain on his main domain:

sub.test1.com > test1.com

Without changing the URL to "test1.com".
Is there a possibility with .htaccess ?

I already tried:

Code:

RewriteEngine    on
RewriteRule    ^/$ /srv/www/virtual/test1.com/htdocs/

Code:

RewriteEngine    on
RewriteRule    ^/$ /test1.com/htdocs/

Code:

RewriteEngine    on
RewriteRule    ^/$ /../../htdocs/

All without success.