It's been quite a while since this question was asked but after the release of the movie "Fantastic Beasts and where to find them" I read an article in which J.K Rowling confirms that Grindelwald was a Seer.
So that could be a reason as to how he already knew Voldemort. It would also explain his short and cryptic conversation with Voldemort slightly better, like when he says "“You will not win, you cannot win! That wand will never, ever be yours—“". He already knew about the outcome of Harry and Voldemort's ultimate showdown.
The tweet from J.K where she confirms Grindelwald was a seer:
He is a Seer AND he was lying.
J.K.Rowling on Twitter, 11:48 PM - 18 Dec 2016