Chris Brunt had earlier missed a penalty for the Baggies, but Tchoyi spared his blushes when he cancelled out Nicola Domenico Sansone's goal for the Serie A side.
The hosts nearly snatched an early opener as Tchoyi just failed to get a telling touch on Graham Dorrans' cross.
West Brom then spurned a golden chance to open the scoring as Brunt saw his penalty saved after Paul Scharner was fouled in the box and this wastefulness was punished when Sansone rifled the ball past Ben Foster in the 43rd minute.
The arrival of substitute Simon Cox appeared to lift the Baggies after the break and the striker sent the ball agonisingly across the face after rounding Nicola Pavarini.
But West Brom did manage to snatch a goal in the dying stages when Tchoyi bundled in Jonas Olsson's cross.