What you’re looking for is the -C
option:
$ man tar
<snip>
-C, --directory DIR
change to directory DIR
Usage, per your example:
$ tar xvzf archivename.tar.gz -C /tmp/testdir/ sampledir/
Also, you may be interested to know that modern versions of tar
don’t require the z
flag to decompress gzipped files; they’ll notice it’s a .gz
and Do The Right Thing automatically.