Pop GA and Ult on tryndamere at the same time! [GAMEPLAY]

So if I have Guardian Angel on tryndamere and I press R too late to activate before taking fatal damage, my ult will be active while I am in revival stasis, meaning that I lose cooldowns on both GA and R on the same fatal damage. This happens every time I play tryndamere because I am a total noob that ults late. I watched a high elo tryndamere gameplay on youtube (Neace or Boxer Pete I think) and learned that Tryn R has a cast time? I assume that what's happening is I press R before taking fatal damage, take fatal damage before cast time is complete, and then I have undying rage while I am in stasis so I revive from GA after 4 seconds with 1 second left of undying rage and lose the cooldowns for both.