Originally Posted by rufus_smith
About an hour into a match, some players have a tendency to stop lifting up off the ground (a little jump or hop) when contacting the ball on the serve, thus contacting the ball surface in a little lower spot and pushing it long. Solution: keep lifting up even when tired.
Bingo. This also translates into what I was going to suggest: focus on bending your knees, because as someone said, you're not getting up enough to bring the ball down.