The phrase "unlikely comeback" means that it is not expected or very unlikely for someone or something to make a successful return or recovery after a setback or difficult situation. Full definition
You don't need 1,500 words on the beauty of the tremendously unlikely comeback win. (sbnation.com)
But it's worth pointing out that unlikely comeback wins that don't happen are also fun. (sbnation.com)
The 2013 America's cup will be long remembered as one of the closest matches in sailing history and one of the most unlikely comebacks in sporting history when Emirates Team New Zealand came within seconds of securing the America's cup; but eventually lost when Oracle's team USA followed a two race penalty and six race -LSB-...] (greenprophet.com)