What I did in mine was to make the AI go at 3 different speed levels based on the distance from the ball. When the ball was very far, or on the other player's side, it would move very slowly if at all. When the player entered the AI's field, it would already go faster. If the ball had gone through half of the field, then the AI would speed up to catch it in time. It lead to some fun matches even though the scoring points were all the same, but that can be solved by introducing some random factors, like for example variating the distance to which the AI should speed up.