Hendry began his recovery by taking the 10th frame on the black after securing the two snookers he needed.
Allen needed a snooker in the next frame but, after successfully drawing Hendry into a foul, the 22-year-old from Antrim hammered the final black into the jaws of the bottom pocket and watched in disbelief as it bounced off the table.
Things went from bad to worse for the former world amateur champion in the next, referee Michaela Tabb calling a foul when his waistcoat brushed a red as he stretched to pot the black while on a break of 57.
Hendry was a 33-1 outsider at the start of the championship and, although he could not argue with that assessment of his chances at the time, the 39-year-old was in bullish mood after his victory.
"I've got to take confidence from a win like this, to be up against it all the way and come through in the end, and to win the last frame in style in one visit is very pleasing," he said. "You only have to look at my record the last two or three years and even I would have written myself off. I'm not down as one of the favourites but I'm easily capable of winning it."
Twice-winner Mark Williams is two frames away from a place in the second round. He leads 8-1 against Sussex qualifier Mark Davis, who won the final frame last night 72-9 to avert the threat of suffering only the second whitewash at the venue.
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