Finally eclipsed the 130 mark with a 131 mph. Whew! I was beginning to doubt my testosterone levels! LOL!
133 I've gotten that a few times, and I think that's as fast as it's going to get. I've applied all 14 stars to my player, two of which are on my serve. I also have the "Ace" skill, which I selected so that my serve wouldn't be a weak point. My fastest non-risk serve so far, that I've noticed, is 124.