You can use the above tool to answer. If player1 wins 30% of points returning, that implies that player2 wins 70% of points serving. Input that number for p2 in the calculator, and check how the winning probability changes as you change p1. As you pointed out, at p1=70% the match proba will be 50%. At p1=65%, proba drops to 27.4%
In general, tennis matches are very sensitive to this point winning probability.
