Firstly, in order to do a "break ace", you have to get the first shot where you get a hit at the diamond of balls, and during this hit, you have to get the (9) ball into a hole first go, and it'll be a break ace, if you take another turn to hit it in, it won't count.
Now then, you're not able to obtain this in Kamurocho as there's no pool place in there, so this has to be in Downtown Ryukyu, in the club "Aqua Sky". Just simply challenge the beginner difficulty guy and try your best to get first shot to break the diamond as it'll not work otherwise.
I've seen a lot of people post /tricks/ to get it first go, but none of them worked for me, what I did was position my cue ball on the spot it spawned on and not move it, then hit it full power right ahead into the front ball. It took 40 or so minutes until I was able to get the Nine in first shot by pure luck, so, if it doesn't work the first few times, just keep at it!