Scotland's Anthony McGill opened up a surprise 6-2 lead over England's Kyren Wilson in the first session of their last-four encounter at the Crucible.
Eighth seed Wilson, 28, was highly fancied to progress after claiming an impressive 13-9 victory over defending champion Judd Trump in the previous round.
But he was knocked completely off his stride by McGill's blistering start, with the world number 39 claiming the first three frames and looking at home in his first World Championship semi-final.
Wilson continued to make uncharacteristic mistakes as the pair shared the next four frames before the Scotsman underlined his dominance, ending the session with a break of 92.
The match continues from 10:00 BST on Thursday - read the full report here.