Hardwood planks are considered dimensional lumber and as such they offer too much instability under the tile, in other words they move a lot and a sub floor that moves a lot, due to seasonal weather, will kill a tile floor.

Rip it out and replace with BC grade ply. What kind of subflooring is under the hardwood, what is the size and spacing of the floor joists and their unsupported span? All this needs to be checked out to see if the subfloor is stable enough to support tile .