Andy Wood,
Why the goal criterion? Every creature might be said to be engaging in goal-directed activity without actually having said goal. Also, what if the very goal of intercepting the ball is not intelligent?

Admittedly, the "mental" aspect of "mental ability" might be difficult to apply to computers. Perhaps it would be an improvement to say intelligence is cognitive ability or facility. Mental abilities can take many forms and can be used in pursuit of many goals, but I think it is the abilities themselves which constitute intelligence. One who has better "mental abilities" will be better at pursuing their goals - whatever they might be - and indeed, better at determining which goals to pursue.