In juniors, Donald Young was up 5-1, 40-15 in the final set to Jesse Levine. He then lost 23 consecutive points, and the match.
But for pros? Easily has to be Ferrer-Almagro this year at the AO. Not only was he up 2 sets to love, but he served for the match 3 TIMES, in the 3rd and 4th set.
I am a fan of Federer, Nadal, Djokovic and Murray. Yes, that's possible.