Ugh, finding quality content for Yuki and Akiho can be a real deep dive sometimes. The pairing hinges so much on that unspoken trust and the sheer, oppressive weight of their circumstances in 'Future Diary'/'Mirai Nikki', you know? Emotional tension is the whole point. Most fics that nail it tend to be on the shorter side, focusing on isolated moments.
One that comes to mind is 'Static Interference' over on AO3. It's a canon-divergence where Yuki's paranoia gets the better of him much earlier, and he ends up kidnapping Akiho instead of Yuno. The entire story is this claustrophobic, tense game of cat and mouse in a safe house, with Akiho trying to logic her way out and Yuki's sanity fraying at the edges. The emotional tension isn't romantic so much as a horrifying dance of manipulation and desperate, misplaced care. It gets under your skin.
Another is a more experimental piece called 'The Observer Effect'—it's told from Akiho's perspective as she pieces together Yuki's diaries after everything. The tension there is all in the gaps, the things he didn't write down, and her cold, clinical analysis slowly breaking down into something like grief. It’s quiet, but it leaves a mark. You kinda have to search for these by filtering for 'Akiho/Yuki' and tags like 'psychological', 'emotional hurt', or 'mutual pining'. The real gems are often buried under a mountain of more popular ship content.