it's not over until the fat lady sings

It's not over until the fat lady sings. (informal) — Еще не вечер; Еще не все потеряно

something that you say when someone is losing a game or competition but you think there is still a chance they might win.

Example: Tony's only two games behind. And as they say, it's not over until the fat lady sings. (CIDI)

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The opera ain't over till the fat lady sings, was coined by Dan Cook, a sports broadcaster in April, 1978.