A mammoth afternoon session between Wu Yize and Mark Allen saw the longest frame in World Championship history completed.
The afternoon session at the Crucible concluded with a bizarre 14th frame, which saw a cluster of eight reds jammed around a black on the edge of the pocket.
It resulted in a lengthy stalemate which passed without a ball being potted and included referee Marcel Eckardt asking the Crucible crowd to settle before Allen eventually fouled the black.
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A gruelling frame was completed in one hour and 40 minutes -- the longest in World Championship history -- leaving the pair tied at 7-7 overnight as the session ended two frames short.
Allen knew he had to hit the ground running after trailing 6-2 overnight to an opponent high on confidence and belief.
The Northern Irishman dug deep to win a scrappy opener with Wu failing to make the most of getting in among the balls before ending on 32.
What followed next was extraordinary with a frame of ebb and flow lasting over an hour.
Wu racked up a 51 break before choosing not to split the remaining reds and opting for a safety shot instead.
The move looked like paying off but Wu missed a key brown and no ball was potted for 30 minutes as a safety battle took charge.
Wu eventually found a red but failed to cut a green into the middle pocket, allowing Allen to find the snooker he needed and eventually take the frame on the black.
The Crucible crowd stood to salute Allen and they were on their feet again within minutes after he compiled a tournament-best 145 break.
Wu looked a forlorn figure in need of the interval until fluking a black that rattled the jaws of one pocket and rolled along the cushion before finding another.
But Wu was unable to take advantage of his good fortune and another error allowed Allen to square matters at 6-6.
Allen maintained his momentum after the interval with a 121 break, his ninth century of the tournament taking him one clear of Zhao Xintong, to lead for the first time.
Another extraordinary frame followed in the 14th, with the black crowded by a cluster of reds and after a lengthy stalemate, Allen knocked the black in.
The frame continued to ultimately make Crucible history, with Wu battling to clinch it and level the semi-final.
Six-time world champion Steve Davis was unhappy with the situation in the 14th frame.
Speaking on the BBC afterwards, Davis said: "In a nutshell that frame is an embarrassment to snooker, and the referees' and the players' association need to try to work out a way that never happens again."