Is there someone you've played who has exceptional fitness that he uses as a weapon? Maybe playing him again can motivate you to get as fit as him.
If tennis is your love, do you approach hitting sessions with determination to come out of a split step every time your opponent hits, get to every ball, and recover as quickly as possible. Is there someone you know that likes to really work hard during hitting sessions with continuous play?
Have you used a heart rate monitor?
HIIT based on reaching maximum heart rate levels can be motivating in that you have a target to shoot for.
Would a change in location help your running?
Is there a nearby park or woods or run to a destination that would make it more enjoyable?
Have you ever found that sprints on a football field where the yard markers quickly disappear make you seem like you are running even faster? (And many football fields are well maintained, softer surfaces than other alternatives.)
Any desire to get really stronger?
Starting Strength by Mark Rippetoe.
Soccer was mentioned above. How about basketball?