Frankly, that would be my first thought, unless I literally meant juice that I just squeezed out of a single lemon. "Sok z truskawki" would also be juice out of one strawberry, so that would be already really strange. 'z truskawek', ok, but mostly adjectives are used, specially on products in the shop.
Well, maybe "sok z cytryny" sounds better than "sok z cytryn" actually and maybe it doesn't have to be literally one - although technically that's what the phrase says.
Actually, maybe "sok z cytryny" sounds ok in a recipe as this is something that you are going to squeeze yourself probably, in contrast to "sok cytrynowy" which sounds like a box of juice on a supermarket shelf. Still, none of those meanings is set in stone, someone else could read those differently, perhaps...
Cóż, może i "juice from lemons" to technicznie rzecz biorąc "sok z cytryn", ale tak naprawdę rzadko kiedy "sok z cytryny" będzie sokiem z dosłownie jednej cytryny ;) Myślę, że można więc uznać, że to praktycznie to samo.
A jeśli chodzi o "juice from lemon", to niestety brakuje przedimka. Albo "a lemon", albo "the lemon", co oczywiście oznacza konkretną cytrynę i jest średnio prawdopodobne. Ale nie samo "from lemon". Pytałem Anglika.
Even the English sentence has "from a lemon" here. For Polish people, that's indeed juice "from a lemon" or "out of a lemon", which takes the preposition "z" + Genitive. "sok cytryny" would sound like "lemon's juice". I think it even makes some sense, but it's not a common thing to say and I definitely wouldn't use it here.
You're asking about Instrumental because that's what's you're used to see after "z"? OK, but that is "z" meaning "with", so it's a different meaning.