The Englishman will now face Luca Brecel, who overcame surprise starlet Si Jiahui earlier in the day, in tomorrow's final.
Here's what happened in matches at the Crucible...
Mark Allen 15-17 Mark Selby
This match was always going to be a blockbuster, featuring the world number two, Selby, and three, Allen. After six frames the duo were tied at 3-3.
Allen managed to pull away by claiming the next three, but it was tied again at 6-6 after 12 battling frames.
'The Jester' then moved into a strong lead after the break, taking the tie to 16-10, on the brink of victory.
That was before Allen valiantly pulled the score-line back to 16-15, before Selby rounded off the result with the 32nd and final frame, just short of 1am.
Luca Brecel 17-15 Si Jiahui
Jiahui started the tie well, moving into an early 4-1 lead after surrendering the first frame to Brecel.
The Chinese hot-shot stamped his authority on the match, storming ahead 14-5 lead after 19 frames.
Brecel then got a grip on the tie and claimed the following 11 to take the lead in dramatic fashion at 16-14.
The duo exchanged the final two frames to round off a scintillating tie.
