Stephen Maguire's frustrations spilled over and his hopes of reaching a first Betfred.com World Championship final took a major hit today.
Opponent Ali Carter, the 2008 runner-up, showed the temperament required to win a Crucible semi-final as he turned a 5-3 lead into a more comfortable 10-6 cushion.
Carter requires seven more frames, and in theory could win through tonight when they return for the third of four sessions.
The guidance Carter has received from master tactician Peter Ebdon shone through as he played a containing game, drawing frequent mistakes from Maguire and winning the psychological battle.
Maguire, 31, twice lost his cool before the mid-session interval, and later questioned the judgement of referee Leo Scullion, his fellow Glaswegian, when a foul was called.
Maguire briefly got to level terms at 5-5 but Carter then found another gear and went to their mid-session interval on the back of a 118 break, the first century in the match.
It was in the early stages of the opening frame that Maguire's temper first surfaced.
Carter played safe and put Maguire in cosy behind the brown, seemingly with a hint of fortune.
Maguire might have taken time over his reply, with various potential routes out of trouble available to him, but instead he immediately got down and smashed the white off a side cushion and into the reds.
It was a guileless shot, and considering it came only 10 minutes into the day's play showed he was struggling to focus.
Maguire again lost his cool in frame 11. Trailing, he took on a long red and missed, thumping the table in anger. The audience gasped, Maguire returned to his seat, and Carter knocked in a frame-clinching break.
Once they returned from the mid-session interval, Carter won a key frame which began with him calling a foul on himself and then saw Maguire quiz Scullion over a shot when, aiming to strike a red, he hit the black. Maguire felt he had hit red first and briefly appeared to question Scullion's eyesight before backing down.
Maguire was 73-15 behind later in the frame, with 51 points available, but he prised the two snookers he needed, added a third, and was on a break which looked set to be a match-winner.
However pink to black was a tricky shot, and when the white clipped a middle pocket knuckle on its way back from baulk, it halted the run.
After a short safety exchange, Maguire was presented with a half-chance of a long black, but he missed and left it for Carter who made no mistake.
A 65 gave Carter the next, Maguire replied with a superb total clearance of 142. The final frame gained added importance and Maguire blew a 43-17 lead, Carter's superior safety tying him up and forcing the errors.
At its denouement, Maguire played a swerve shot to strike the yellow, left Carter a clear chance, and he swept up the colours to the pink to seize his four-frame lead.
The pair are vying for the right to take on Ronnie O'Sullivan or Matthew Stevens in the final, which begins on Sunday. O'Sullivan was resuming this afternoon armed with a 5-3 lead over Stevens, after a disjointed opening session to their match last night.