Here's the earliest "sphinx/thinx" pun we've been able to dig up: ""The mysterious sphinx: who know what he thinx?"  From Grip, 1891.

But there have been some echoes along the way:

The reporters go up to the sphinx

And demand to know just what it thinx

     Of the Philippine Isles;

     But the sphinx only smiles

And replies: "That's a secret, by jinx!"

—The Indianapolis News, 1900

That granite-faced dub called the Sphinx

Has missed many thousands of drinx,

     But equals, perhaps,

     A great many chaps,

In the number of thunks that he thinx!

—William Frederick Kirk, Fleeting Fancies, 1904

Even the enigmatic old sphinx winx as he thinx of the most exhilarating of drinx.

—The Capuchin Annual, 1949

The toffy head waiter with the face of a sphinx

These are a few of my least favorite thinx.

—Bulletin with Newsweek, 1981

The Publisher mysterious enigmatic Sphinx.

The Editor in oracles of rejection thinx.

—Marjorie Bovee, A Soul in Search of Itself, 1999