You have a problem there my friend. I can assure you you don't want to use anything that is going to overlap the tile on the top step. If you do that will be one dangerous sucker. You never want to raise the lip of a stair tread. It is a serious trip hazard and is unlawful in some places.
I would have filled the riser depression with plywood so as to flush the front of the riser with the front of the stair nose. Then apply cement board to the riser and install tile to that riser only. This way the upper floor tile would lap over the edge of the vertical tile. That edge would either be made of bullnose tile or metal tile edge. That juncture is then grouted to provide rigid support for the floor tile edge above.
People naturally roll their feet over the edge of steps, if the floor tile isn't supported firmly it will chip and spall off over time.
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