Ted Lilly provides a nice example of what happens to a pitcher when he tires. Note, first of all, that hits against Ted come in a very concentrated part of the strike zone:
In that red area, his release velocity tends to be slower:
Compare that to the outs he gets on his fastball:
When Ted throws his fastball fast, around 90 MPH, batters can't handle the pitch as well. When it drops to the mid 80s, they cream the pitch. From looking at the movement of his pitches, the quicker fastballs stay up higher in the zone, likely preventing the hitters from squaring them up as well. When Ted's velocity drops, however, the pitches come in pretty straight, waiting for a major league hitter to pounce.