it's not over till 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's not over till it's over!" It's not looking likely that the senator will be elected to another term, but it's not over till it's over.

Inf. It is not over yet and will not be until the event has completely played out. It looks like we have won, but it's not over 'til it's over! They haven't won the game yet. It's not over 'til it's over.

Don’t count on an outcome until the very end. Exactly where this twentieth-century adage comes from is not known, nor is the more widely quoted it’s not over till the fat lady sings. There are various theories about the provenance of the latter. A fairly logical one is, the opera isn’t over till the fat lady sings, which appeared in the Washington Post on June 13, 1978. It actually was a reply from sports editor Dan Cook to “The rodeo ain’t over till the bull riders ride.” Another theory is that the saying alludes to Kate Smith, a popular singer in the 1930s and 1940s with a figure as ample as her big voice. Whichever, the phrase has quickly become a cliché.