" From the quarterfinals on, the tournament is single-elimination. "

"Single-elimination" is not an adjective. Could it be a mistake?
It can be an adjective: a single-elimination tournament/round.
I understand that a noun could function like an adjective if it is used a premodifier of another noun. But, "single-elimination" in my example is used without modifying any noun.
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Yes, it can function just like an adjective. It is a predicate adjective of 'tournament'.

It functions as a predicate adjective as in the sentence 'she is beautiful' (beautiful - a predicate adjective).