Very exciting to hear about commentary from someone experienced and with a passion for the game.
I would agree with Tintin5, there is something very jarring in sports games with commentary when you keep hearing very specific lines being repeated again and again.
Keeping the commentary generic for regular use and having specific/memorable lines appear very infrequently is important.
Also, keeping things understated I think will help. In real tennis commentary most of the talking occurs when tennis isn't being played and it's usually quite chilled out and conversational unless immediately following an intense point or great shot.
Another issue I've noticed in certain other tennis games, is that every point is immediately followed by a 3 second sound bite, like 'amazing shot down the line!' then radio silence, or 'he's unplayable at the moment'... then radio silence until the next point. In real life there is always a bit of follow up to the initial exclamation, like 'let's see if he can build on that return now' or 'he's being very patient in these rallies, biding his time until he sees an opportunity to attack'. That would definitely add to the realism I feel.
Obviously I don't know how easy/hard it is to do any of this, but just giving a few suggestions in the hope they might help in some way.
I really like the idea of referencing the tournaments as much as possible without treading on legal toes. If you're able to say things like 'SW19', 'Flushing Meadows' etc it would be great. I think the level of involvement you have in the commentary at this early stage will hopefully result in something that will integrate really well. Exciting!