Mix up the type of serve that you hit to prevent your opponents from getting into a rhythm. Every competitive player should have multiple types of serves in their arsenal. You should vary your serves in terms of spin, target, and pace to always keep your opponents on their toes.
Spin: Mix between high, loopy topspin serves, hard and low flat serves, and tricky slice serves that curve in the air. You’ll also find that some opponents struggle to deal with one spin over another, so make sure that you’re giving your opponents a variety of spins to test them.
Target: Make sure that you’re mixing up the spot that you serve to. Mix up giving your opponents forehands, backhands, and serving directly towards their body. Doing this is likely to expose weaknesses that you can exploit.
Pace: Vary the pace that you hit with. Harder isn’t necessarily better on the serve. Some opponents like redirecting the pace of a hard serve, but struggle to create their own pace off of a soft serve, and vice versa.
Every opponent is different, and weaknesses will only present themselves if you look for them!