John Higgins fought back from one of the worst Crucible sessions of his career to keep his dream of winning a fifth World Championship title alive at the Crucible.
Higgins was in desperate form for much of the morning session against David Gilbert but went on to score a tournament-high break of 143 as he reduced the overnight deficit to 13-11.
It was a gutsy performance from Higgins after Gilbert had started the day by winning three straight frames - including a break of 125 in the opener - to turn a 5-3 lead from the first session into an 8-3 advantage.
But for a missed green in the next it could have been 9-3, but Higgins characteristically dug deep to win the next two and haul himself back into contention.
The Tamworth player responded magnificently, with a break of 72 winning the penultimate frame and a nerveless final black sealing a 10-6 advantage ahead of their return for the evening session.
The evening's action began in bizarre fashion when the referee ordered a member of the audience to leave for apparently shouting something following the initial break-off shot.
Gilbert continued to show few signs of nerves as he extended his lead to five frames by taking the opener.
Higgins, who still seemed far from top form, pulled one back after a lengthy safety battle and, after Gilbert won the next with a 75, the Scot brilliantly cleared the colours to reduce the deficit again to 12-8.
Higgins' stunning clearance of 143 also saw the tournament reach 86 centuries for the fortnight, equalling the record.
Gilbert appeared unfazed and responded immediately with a break of 91 to re-establish his four-frame lead.
But the underdog's nerves were clearly on show in the final two frames of the session, which Higgins battled through to win and reduce the deficit to 13-11 going into Saturday afternoon's concluding session.
Trump begins to assert
Tournament favourite Judd Trump took control of his semi-final with Gary Wilson during the second session on Friday afternoon.
Seeking a first world title, the reigning Masters champion leads 9-7 after finding an extra gear - though Wilson's hopes were enhanced as he composed himself to win the final frame of the day.
Having resumed at 4-4 following a tight opener in which Wilson's 140 break was the highlight, it was the underdog who poked his nose in front.
But after the following two frames were shared, Trump kicked into gear with breaks of 73, 123 and 75, before making it four in a row by taking the 15th frame.
Leading 9-6 with one frame of the afternoon remaining, Trump was looking to move into a commanding lead but to Wilson's credit, he compiled a frame-winning break to cut the gap to two.
Semi-final schedule
FRIDAY MAY 3
Morning Session (10am)
TV Coverage: Eurosport & BBC
- David Gilbert 10-6 John Higgins
Afternoon Session (2.30pm)
TV Coverage: Eurosport & BBC
- Gary Wilson 7-9 Judd Trump
Evening Session (7pm)
TV Coverage: Eurosport & BBC
- David Gilbert 13-11 John Higgins
SATURDAY MAY 4
Morning Session (10am)
TV Coverage: Eurosport & BBC
Afternoon Session (2.30pm)
TV Coverage: Eurosport & BBC
- David Gilbert v John Higgins
Evening Session (7.30pm)
TV Coverage: Eurosport & BBC