This is my dahlia

Well, you're right that the leaves are the same. And I would say they are indeed the same plant, but are you sure your dahlia plant is in fact a dahlia? I've never seen a dahlia with leaves like that. Of course, I'm no expert, so maybe wait to see what others have to say about it.

It kind of resembles a hydrangea, but I wouldn't have guessed dahlia. Have you seen blooms on it yet?

I'm sorry, but those don't look anything like dahlia leaves to me. If you live in zone 6 or lower, chances are the cold killed it. I'm in zone 5/6, and I have to dig them up every winter. Even if you live in a warmer climate, lots of things could have done it in, like too much moisture. An overturned pot, extra mulch or digging them can help overwinter in the warmer areas.

Hi there,
going by the foliage, its shape and position on the stem,
it could be something from the Labiatae famili, like Coleus, Salvia or Perilla.
So, probably some weed, but my guess is not a very aggressive one. You could wait for flowers, that would make an ID much easier.

I am having the same problem right now. I planted some dahlia tubers in a pot on my front porch. I thought the tubers looked old but I thought I'd try them anyway. When they didn't bloom or even come up then I planted gardenias in there instead. Now I've got this one long stem with leaves on it and I don't know if it's a dahlia or a weed but the leaves look just like the leaves that you have pictured. what yours end up being? Was it a weed or was it Dahlia? Thanks!