...Xerath.

So when I play Xerath, pretty much only enemies who I have faced before causing trouble to me are Ahris.

Before she reaches lvl 6, it's all fine but then she gets those 3 dashes...

So how you'd suggest to counter Ahri in teamfights and laning (as Xerath)?

Get MS from quints and Masteries, buy 2-3 dorans during laning, as well as some MRes - HP counters true damage, MRes ... Well it's obvious. Utilize basic MS (from quints and masteries) with boost from abilities to dodge her skillshots and you should be fine.

the problem is that xerath and ahri can be both countered by mobility. Since you hawe less of it your only chance would be utilizing the range and predicting ahris moves.

Well basically use your ult right after she dashed, and wait for her next dash, to ult again, and do the same again. Since she got a really short delay on her dash so she cant dash immiediatly.

Aaaaaand you really think that xerath can kill ahri with 3 dorans before she kills him? I don't think so.

Aaaaaand, do you really think midlane is about getting kills? I don't think so. =P

That being said, I can't see the problem tbh. Insane range for farming. Wait for the gank from jungler.

i don't play xerath but i think that you should try starting with 3 doran rings... She is mostly dangerous after level 6 when she gets her ulti... Another solution is το stay under tower and farm minions there! Let her push and if here is an opportunity to gank her... Just do it! Even though after level 6 he she has flash and her ulti she can go away easily

You have the best long-range champion, and you ask for countering Ahri?
Just dodge her skills. Predict. Move. Poke her with your skills.
And you have stun, havent you?

Xerath might have have the longest range of all ability power champions but well if you notice his package of skills a little carefully you will notice that he is made το fullfill a different role than ahri! Ahri right now is maybe the best mid champion! She is not mana hungry, she farms minions pretty fast and her ulti gives her high mobility and good damage that she can use το hunt enemy, escape or even towerdive and leave from there fast! She's a real menace... Never underestimate her... One more thing that she has as advantage is that mistakes on her can be many times forgiveful

Yup, I played agains her at mid, prety skilled and tricky opponent, but still dodgeable and predictable.
And Xerath is like an artilery, standing and shooting far away, probably Wards would be the "counter" you want against Ahri, hmmm?