On the ball
This American expression dates from the early 20th century. Its origin is baseball where pitchers would put something on the ball to make it swerve and hitters would endeavour to keep their eye ‘on the ball’. By the 1930s, on the ball was being used figuratively to describe anyone who was keenly alert and competent. The expression in this figurative sense was then exported to Britain and elsewhere by the mid-20th century. The associated expression ‘keep one’s eye on the ball’ in the figurative sense, also dates from the early 20th century and means to pay attention and keep focused, which of course is fundamental to all ball sports as well as to life in general.