TE2013, like real tennis, requires time & practice to master. And like real tennis, if your opponent is too strong for you, you'll learn almost nothing & won't progress.
It's really best to raise the difficulty bit by bit, that's why there's SubLevel. So if right now your opponent is Junior-1, try to set its sublevel to 5 instead of 1, and if it's still too hard, set it to 10, or a bit harder if it's too easy ; just find the difficulty you're comfortable with, to win about 60% of sets, and then when you start to win most sets, raise the sublevel by 1, and so on.
For the average person, I think it needs at least 10 hours of good training before being able to beat the Junior-1 with equal skills and the default helpers.
Bonus note: there's no shame in playing in low difficulty levels, the important is to have fun & an interesting challenge..!