John Higgins produced a very out-of-character reaction to a succession of frustrating moments as he slammed his cue down on the ground at the Crucible.
The tension was palpable throughout the match to that point as both legends of the game and members of the famous 'Class of '92' exchanged brilliant breaks, cagey exchanges and a few misses thrown in too.
Higgins found his frustration beginning to boil over after a string of surprising misses to allow O'Sullivan back into frames that looked beyond him, and it culminated at the start of the 23rd frame of the pulsating match.
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The normally composed Scot whacked his cue against the iconic Crucible carpet with a face of thunder after he missed an attempted long red to the corner pocket that would have got him on the board when 13-9 down in the best-of-33-frames encounter.
There was a loud gasp from the audience in the theatre - mostly out of pure shock that one of the calmest players on tour had got to this level of frustration with his game on the biggest stage the sport has to offer.
The surprise was reflected in the Eurosport commentary booth as Higgins strode over to his chair, still visibly raging at both the miss and no doubt, his very uncharacteristic reaction to it.
"Well, there was something that made him furious!" explained Neal Foulds, on commentary for Eurosport.
"That is an absolute collector's item, seeing John Higgins get that annoyed at the table.
"That is the best way I can describe it: he's furious!"
Dave Hendon added: "He just could not stop himself, could he?
"Yes, it is a worrying sign for his supporters. It is an accumulation of what happened in the last frame and then at the start of this one."

Watch as O'Sullivan asks referee to intervene over fan distractions

It is the fifth occasion in O’Sullivan’s illustrious career that he has occupied the world No. 1 ranking, and the first time since 2019, as he continues his pursuit of a record-equalling seventh world title.
The dominance of the legendary trio - O'Sullivan, Higgins and Mark Williams - continues to be utterly remarkable. Higgins has four World Championship titles to his name, while Williams has three.
Judd Trump joked about the "World Seniors Championship" he has found himself in the middle of ahead of the final-four clashes at the Crucible.
"I've ended up stuck in the middle of a World Seniors Championship," Trump wrote on Twitter.
"Super excited for the semi-finals, though, still battling it out with three legends of the game!"
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