it ain't over till/until it's over
The final outcome cannot be assumed or determined until a given situation, event, etc., is completely finished. Usually used in reference to organized competitions, such as sporting events, political elections, and the like. A: "We're down by six points with only two minutes remaining. There's no way we can win now!" B: "Hey, it ain't over till it's over!" It's not looking likely that the Senator will be elected to another term, but it ain't over until it's over.