Where tennis players should be focused and alert (2,3,4)
I believe the answer is:
on the ball
'alert' is the definition.
(on the ball can mean alert or paying attention)
I can't explain the rest of the clue.
(Other definitions for on the ball that I've seen before include "concentrating here during Lottery draw?" , "Ready to take initiative" , "Alert to new ideas" , "Wide awake" , "Alert - informed" .)