I had a background vocal session the other day that went longer than I expected it to. I began to physically tire. As is my habit, I tried to make a mental note to share with you what helps me.
At one point, I had to hold some high, long notes on that pesky vowel sound "e". I usually don't have much trouble, but my breath support was lagging due to my physical fatigue. What to do, what to do?
I made a conscious decision to move my jaw sideways in a very slight little chewing motion. NOT TOO MUCH, or it would cause a loss of control.
What that did was to allow my "e" vowel to relax more vertically. Voila'! It worked! I articulated the syllable, shifted my breath support/control into gear by stretching tall and squeezing from the pelvic floor, pulled the "e" from my voice path, and it was smooth sailing. It worked for other vowels, too.
If you are doing master lead vocals in the studio and you're too tired to sing, in a word...DON'T!!!! You possibly can get by with tired demos and worktapes, but not master leads. The most important part of a master [a master project needs to be ready to pitch to labels, to sell and to be given airplay] is, after all... the vocals.
Let me know how these techniques work for you!