Do you need to open the chest with key in a boss lair in order to see a nemesis portal spawn ?

I've been quite unlucky with the nemesis portal, and i wondered if i had to open the chest to see the portal spawn, i've been using a lot of key, often for loot i don't need or already have. I wouldn't open them as much if the nemesis lair could appear without using the key. Do any of you know ?