take (one's) eye off the ball
To fail to remain focused on or motivated about some goal, objective, or matter at hand. You can't take your eye off the ball just before this merger deal is supposed to go through, not when there is so much at stake! I was on my way to earning my PhD, but I really took my eye off the ball toward the end of the program.
take your eye off the ball
If you take your eye off the ball, you stop paying attention for a moment to something you are doing, and as a result something bad happens. You can't take your eye off the ball for a moment on this project. Compare with keep your eye on the ball.