Switching targets from player to mob and back

I run pretty regularly with 3 other people with whom I've been gaming for most of the past 10 years and the 4 of us can pretty much do any content we want- gets a little butt-clenchy at times but that's part of the fun I recently acquired and leveled up the Valiant Warden skill and though I think it's a nice skill I do have a bit of a problem with it. When I target the tank with the appropriate F key I pretty much keep him targeted and keep Valiant Warden refreshed while at the same time casting AoE's on the mobs around us. But when I target a specific mob for whatever reason and then try to retarget the tank with the appropriate F key it doesn't work until I untarget the mob I have targeted and then hit the appropriate F key.

 

Yea, that's one of the more annoying aspects of our targeting.
Having to hit Esc to get rid of a targeted mob just so you can use your Fkey targets - is a PITA.