You must definitely write : a practice, but to practise.
Exactly when as you write : an advice, to advise ; or : a device, to devise...
The reason is etymological, from French (and don't ignore that two thirds of the English vocabulary comes from over The Channel, it explains numerous oddities in English spelling and pronunciation - in fact regularities in French, or rather old French).
Take for instance :
a) advice : the old French was "un advis" (the "d" is now dropped) ; it was pronounced, and still is in Soutern France : avisse (avi in the North).
b) advise : the verb was "adviser" (the "d" is now dropped, too) ; it is pronounced avizer
The French rules ensue themselves from Latin.
in practise est donc incorrect ; the confusion comes from the USA ; the immigrants have not always been good grammairians, and they still have no reference like Oxford or l'Académie