The 'Belgian Bullet' looked on course to be knocked out of the tournament by the young debutant who is looking to become China's first world champion.
Si took a commanding 11-5 lead from the morning session and was ruthless when play resumed, breezing his way through the opening three frames to lead 14-5.
At times his snooker was breathtaking with the Crucible crowd cheering as the lowest-ranked player of the tournament pocketed breaks of 132 and 97.
No player has come from nine frames behind to win at this stage of the tournament but Bracel staged the unlikeliest of comebacks to keep his slim hopes alive.
A suddenly nervous-looking Si missed several pockets as a resurgent Brecel won five frames on the bounce to claw the score back to 10-14.
Si, who had to win three qualifying matches to reach the Crucible, began to crumble and after missing a straightforward green at 34-53 in the final frame, punched the table in disbelief.
The match will go to a final session on Saturday with the winner to play Mark Allen or Mark Selby in the final on Sunday.
