The phrase "improbable comeback" means the surprising return or victory after a difficult situation or when it seemed unlikely to happen. Full definition
The Boston Celtics and Houston Rockets are halfway to completing improbable comebacks from 3 - 0 deficits. (sbnation.com)
The 2004 Boston Red Sox staged the most improbable comeback in baseball history and liberated their long - suffering nation of fans (si.com)
Enter David, in the form of Team Grazioso who literally minutes prior had completed one of the most improbable comebacks in All Sports Series History... So, naturally Team Grazioso sprung out to an early lead! (allsportsseries.com)