Si Jiahui was leading 14-5, but Luca Brecel completely turned the match around and reached the final through a 17-15 win.
In the semi-final between favourites Mark Selby and Mark Allen, only 13 of the scheduled 16 frames had been completed after two sessions, but both men are still reasonably back on track after the Saturday morning session (11-10 for Selby).
Si Jiahui 15-17 Luca Brecel
For two players who until the current tournament respectively never played or won a match at the Crucible, Chinese Si and Belgian Brecel put on an unprecedented spectacle.
Virtually every frame was to be enjoyed and savoured. Si started the Friday morning session with a 5-3 lead, which he extended through 6-4 to 9-4 and 11-5.
The long pots and bold, but well thought-out, choices Si made put Brecel's back against the wall. In the later session, Si ran out another three frames with virtuoso play, but then Brecel hit back.
The Belgian, who already cleared a two-frame and four-frame deficit in the previous rounds and finished with a 7-0 run against Ronnie O'Sullivan, narrowed the gap to 14-10.
Much to the delight of the 1,000-strong sold-out arena, which has traditionally sided with the underdog. Hoping for another top nine frames.
He needed three more for a win, but saw Brecel extend his streak of frames won to as many as nine and so both players went into the midsession interval of the four session at 14-14.
The semi-final was reduced to a sizzling best-of-five in which Brecel got to 16-14, Si fought back one more time but then finally had to bow to the Belgian: 17-15.
Mark Selby 11-10 Mark Allen
It could be a very long Saturday for Mark Selby and Mark Allen. Indeed, in the Friday afternoon session, both seeded players only completed five frames in just under three-and-a-half hours of play.
Allen built his lead from 5-3 to 6-3, completing a four-frame streak. Selby did not let that get the better of him and so a war of attrition unfolded with a lot of tactical play and frames with a playing time of over 45 minutes.
An incredible contrast to the semi-finals in which the average time for a pot sometimes stood at just 12 seconds. The Crucible did not have to sit on the edge of their seats, but nevertheless were entertained with a piece of 'theatre' put on by the two main players.
The experienced Englishman and Northern Irishman joked back and forth after several 'safeties', stalemates and 'reracks'.
At 10am, the men returned to the table and Selby confidently built his 7-6 lead to 9-6. At that point, Allen triggered a six-frame streak and came back to 9-7.
At 10-7, Selby was on the verge of a short evening session for himself, but via 11-9, it became 11-10 in favour of the four-time world champion after three-and-a-half hours of snooker.
Final
The eventual winner will need to win at least six more frames in the evening session from 7pm and will face a player with a totally different style of play in the final on Sunday.
Moreover, despite the gruelling comeback, Luca Brecel has at least one more evening of rest and, moreover, played only eight frames on Sunday. The Belgian can watch the evening session in peace on the bench, while the maximum 12 frames from the final session are not expected to be decided just in a few hours.
In the first semi-final, frames lasted an average of 15 minutes, where both Marks needed double the playing time. With Allen's form from the second part of the morning session, it could be some serious night work in Sheffield!
