On my Late 2011 MacBook Pro (MacBookPro8,2), I was able to boot into Recovery's Single User Mode on 10.13.6 by continuously holding down Cmd+R at boot and then a few seconds into the grey screen (but before anything else displays) moving directly to Cmd+R+S, not releasing the first two keys when transitioning. Might take a few tries as the timing is inexact.
To describe the sequence in more detail, my MBP first boots to a solid-grey screen along with the startup chime. After some amount of time (presumably loading) on this grey screen the FileVault decryption fields appear. Here is generally what it looks like once those fields are on-screen:
(image sourced from Rich Trouton)
Before the fields are on-screen it's exactly the same solid-grey screen but without any other elements on top of it (Apple logo, login items, etc). It takes some time for the completely grey screen to move to what is pictured above. I add S to my key combo between this populated grey screen and the completely solid one. And I noticed that to get it to work consistently I had to add S closer to the appearance of the pictured populated grey screen than to the appearance of the initial solid-grey screen.
I believe newer MacBooks no longer feature a grey screen when FileVault is enabled and instead opt for a black one which eventually becomes a much more attractive login screen with a wallpaper background image. If I had to guess this approach could still work for these MacBooks but I don't have one nearby to experiment with.
The basic generalizable idea is that you hold Cmd+R continuously, press the power button, and then as soon as recovery mode starts you switch to holding Cmd+R+S continuously without releasing the first two keys. The annoying part is that if you switch to the second combination too late then recovery mode stops looking for it and boots into regular recovery. Too early and recovery never boots at all and you go to single user mode on the normal partition. Additionally annoyance is created by the fact that, at least on my MacBook, there is no visual indicator where recovery starts booting and you need to add the S. If recovery's loading bar appears it's already too late to add S.