Allen will face old foe Stuart Bingham in the final after the Basildon potter came from 4-2 behind to complete a 6-4 win over home hopeful Ding Junhui.
A break of 97 helped Bingham close out the match after runs of 87, 55, 46 and 59 had kept him in touch trailing 2-0, 3-1 and 4-2.
Allen had made some derisory comments about Bingham before Bingham beat him in the 2011 Australian Goldfields Open quarter-final claiming that he had no "bottle" after blowing a 12-9 lead against Ding Junhui in the World Championship quarter-finals in losing 13-12 three years ago.
Bingham went onto win that title, and will hope for a similar outcome against Allen this time.
Selby had raced into a two-frame lead courtesy of breaks of 70 and 116 but Allen took a tactical frame three courtesy of runs of 52 and 17.
However, Selby looked like he was running away with it as he won the next two to open up a 4-1 lead but Allen again cut his advantage with a break of 68.
Allen got his hand on the table first but could only make 17 in frame seven and, after a lengthy tactical exchange, the ‘Jester from Leicester’ secured the frame with a break of 71.
However, Allen reeled off the next four frames to secure his spot in the final.
Frustratingly for Selby, he managed to construct a break of 63 in the deciding frame but ran out of position and allowed Allen in who put together runs of 40 and 27 to secure the win in the best of 11 encounter.
"It's good to get one over on Selby as he does it to so many players," said Allen. "I started poorly so I had to dig deep and I played much better after the interval. In the last frame he looked as if he was going to close out the match in one visit so I'm very happy to pinch it on the last black.
"The last four frames was the best I've played so far in this tournament, but I can play better and hopefully I'll do that tomorrow."