We know Ranma falls first and hard, so what about Akane? When does it start? unlike Ranma, I think it's a couple of things adding up in the background... and why wouldn't it start... here? walk with me
Doctor Tofu is kind to Akane, but I'd say the main reason she had a crush on him was: he was the one male figure outside of her family who made her feel safe. Taking care of her injuries would feel like a form of protection, and Akane wants to feel protected.
It's not just that Akane shows her fiery personality in fights... I recently noticed that while other fighters can show themselves cool and confident in the face of danger (like Ranma). But with Akane, she fights as if she never feels safe in a fight, you can see it. Even when she's the strongest and is winning every morning...
Winning every day could have made her approach these guys looking more "confident" or "relaxed," but she's never relaxed. She always sends Kuno flying, but notice how there's an air of uneasiness (even if she sees him as a buffoon) that doesn't disappear until Ranma comes into the picture.
Ranma showing up as a girl helps Akane relax and reach out as she's too used to being harassed by guys (so she keeps her distance). But even if she finds out his secret in the worst way, she doesn't beat his ass until he's picking on her.
Akane is the one offering the friendly match (connection) but Ranma is also doing something likely no one has ever done with her before: he's being soft, maybe even tender, with her... making her relax. he's making her feel safe (Ranma doesn't fight any other girl like this)
The bathroom incident makes her feel afraid, but when boy Ranma shows himself again, it's clear by the way she stops any attempt of violence to study him (and argue childishly) that the fear is gone. She's tested in the worst way (a way that plays into her worst fears about men, which is hard to shake)... but this tells you she still feels safe, even if she doesn't realize it.
If someone like Kuno had made fun of her proportions, Akane would've beaten him up and moved on. The fact that she's still thinking about it long after the fact tells you he's already stirring something (it's even connected with Ranma thinking about her because he too is also feeling something)
Part of Ranma "falling first" is that he sees her best very straightforwardly from the get-go (and is confused after). With Akane, she isn't even sure of what she's seeing, she's still dealing with her complicated feelings connected to the doc and constant harassment... but she's already interested.
It's obvious that Akane lives rent-free 24/7 in Ranma's head from the moment he meets her, but Akane is not exactly unaffected. It isn't accurate to say he only annoys her at this stage. She pays attention, confides with him, worries, goes after/covers for him... that's interest
You need to pull a rope from both ends to create tension.
Romantic tension requires both parties to feel something, and they already have plenty when Akane tries to help Ranma, and he is protecting her during the fight against Ryoga. Even if she still hasn't sorted out her old crush...
Akane wishes for and values normalcy (she actually has a life) but she only feels normal when compared to the clowns that arrive after Ranma. It's clear by the way her classmates see her that she's an extraordinary girl...
She loves martial arts, but doesn't have the sort of ambitions the insane fighters around Ranma have. She sees it more as a fun outlet, comparable to having a favorite sport. She only holds on to power when someone pisses her off (spite lol) but always chooses normalcy over it
Ranma brings both craziness and safety into her life. You can really see that with Kuno and the guys challenging her: Akane is comfortable supporting Ranma as he navigates the craziness around him, but she isn't comfortable when SHE is at the center of it
Ranma either stands by her side during the madness or straight-up redirects the focus on him (starting from the moment Kuno throws that rose to Akane and Ranma is immediately by her side, ending taking the challenge against Kuno himself). Akane might complain about Ranma "fighting her fights" here, but she quickly gives in to his protection (unless someone is pissing her off... spite, she's just like me fr etc)
In true gag fashion, when Ranma picks on her for her attention, he’s both giving her an outlet and making fighting, which she connected to danger and harassment, a very safe and childish thing. It allows her to relax in a way she hasn’t before he meets him.
When Ranma complimenting her smiles gets to her to the point she's still thinking about it hours after, or that Ranma essentially saying he likes her better as her true self makes her genuinely happy... it doesn't come out of nowhere. She's already been feeling something for him