Why is it "practitioner" and not "practicioner"?

This is one of those questions that just pop into your head when using a word; in this case, "practitioner".

The OED defines a practitioner thus:

practitioner noun

a. A person engaged in the practice of medicine; a physician, surgeon, pharmacist, etc.

Given that a practitioner is one who is 'engaged in the practice' of something, why is it not spelt "practicioner"? Is it to do with the practice being practised?

Solution 1:

Originally, it was spelled practicioner. OED provides examples of this spelling as recently as 1735. This was probably the original spelling based on its etymons: practician n.; French practicien

1735 In Physick there are, and have been many happy and lucky Practicioners who knew not so much as the Christ-cross Row.

• South-Carolina Gazette · 1732–1775.

The change to -tion was likely part of a switch to the Latin suffix which uses a t, and which became a standard spelling for such words that had obsolete forms spelled -cion. OED's entry on coercion offers a foot-note clue.

The current spelling [coercion] is deceptive, suggesting formation < coerce + -ion. This no doubt led to the retention of the c when all other words with the mediaeval spelling -cion, were altered to the Latin type in -tion.

Wiktionary confirms this explanation with added detail:

The Middle English -cioun became -tion in Modern English under the influence of the Middle French -tion and original Latin spellings.

The two remaining exceptions are "suspicion" and, as discussed, "coercion."

Solution 2:

Just to support RaceYouAnytime's answer, and the explanation of analogy, I thought I would post to add an example of a similar spelling I found recently in an old book, The life of a satyrical pvppy, called Nim who worrieth all those satyrists he knowes, and barkes at the rest, by T.M. (accessed from Early English Books Online).

It uses the spelling "Physitians" for "Physician's". The Oxford English Dictionary lists a number of other variant spellings of "physician" that use "ti" instead of "ci". Of course, "physician" is related to words like "physical", and doesn't have any "t" in its etymology.