Difficult colleague

[Songbird54321]

I have not long returned to work from maternity leave. There are quite a few new staff, most of whom are lovely. There is one however who appears to massively dislike me. I don't know her at all so find it a bit odd but can only put it down to her being on a team where a woman used to work who I didn't particularly get along with. She has now left but they are still friends.
I don't particularly have an issue with her disliking me, that's up to her, but it has caused a couple of issues work-wise.
I am senior to her but have little to do with her. On the couple of things we have crossed paths on, she has blatantly ignored my requests/emails and I have heard her whispering to her team mate about emails I have sent, all very childish. This is where my issue lies. I manage a few clients. She is an administrator. Any work for my clients which fall into her area she needs to process.
It's at the point I have had to escalate to the area manager as it will effect our relationship with them massively. They are one of, if not the biggest, client we have.
I'd quite like to call her out on it, but having not been back long I don't want to rock the boat too much.
What would you do?

She sounds incredibly childish, but unfortunately you will have to find ways to work with her. I wouldn't go to the area manager unless it is absolutely necessary, I would personally try to be incredibly nice to her in front of everybody (so that when she talks shit behind your back, she looks bad) and, well, micromanage stuff that she needs to do for your clients until she hates it and starts doing her job faster to limit your interactions/ micromanaging. All interactions with her probably shouldn't be limited to just emails (it is easy to ignore them), maybe you could try coming up to her desk/giving her a call several times a day to ask whether she has already done A or whether there has been any feedback on B. And don't forget to be very nice to her on the surface (while maybe feeling stabby deep down!).
I would also recommend checking out Jennifer Brick's videos on youtube, she has a lot of guides on how to deal with difficult coworkers. I have found them really useful.

Re being nice, I would also "pretend" that any issues she might have caused with your clients already have been "innocent mistakes" on her part and not something deliberate and make sure she knows that's what you think.
I would consider calling out or any sort of confrontation only after using the being nice & micromanaging strategy for a while first

Personally, I would speak to her directly first. Be specific about the problems and what she needs to do. Don't be afraid to discuss her unprofessional conversations.

I would also let my manager know that there is an issue but I'm going to handle it.

I'd quite like to call her out on it, but having not been back long I don't want to rock the boat too much.

It's not rocking the boat or calling her out. It's just having a very simple, professional converation about expectations.

Don't fuck around with playing games or pretending, just communicate clearly and treat people they way you want to be treated.

Thanks for the replies.
I do talk to her face to face as well but as a company rule work requests have to go into an email. I think I've been nice and professional but will keep that in mind.
With regards to escalating to the area manager, I haven't named names, just that xyz has not been done and could cause him some issues at his meeting later this week with them but that I have requested it is sorted.
I think I might go down the route of asking if she needs assistance on what I have asked her to do rather than going in with 'what's your bloody problem' and see where we get.
Her friend I used to work with didn't really like me (it was mutual) but we worked together just fine and were professional. I get along with pretty much every other person in the company so I'd like to think I'm not difficult to get along with. She's very young though so perhaps just needs to grow up