There are things a player can do to improve their focus besides "hitting a lot of balls."
A player can watch less of their ball standing still after they hit and incorporate movement. A player can watch their ball into their strings to keep the head still and disciplined. No you will not be able to see the ball, but you should see a blur. That is good discipline.
A player when the ball is coming towards them can say HIT BOUNCE HIT to stay focused. This is what I do to help remove distractions and increase my ability to time the ball. Or as the above poster mentioned watch the bounce closely.
Former USPTA Teaching Professional
Volkl Tour 10 V-Engine Mid/Luxilon Big Banger