Ali Carter set up a semi-final clash with Shaun Murphy with a 6-3 defeat of John Higgins marred by controversy.
At 4-3 up, Carter was adjudged to have fouled when failing to make contact with the yellow, only to insist that he had in fact touched it, which in turn resulted in the referee reversing her decision.
Television replays appeared to confirm that there was in fact no contact, begging two questions: why did Carter insist that there had been, and regardless of that, why did the referee take his word for it?
It came at a decisive time in the match, Carter going on to win the frame to lead 5-3 when the scores could have been levelled, and we may not have heard the end of it.
Carter had earned a 3-1 interval lead in a suitably deliberate opening, and his lead was 4-1 after he edged the fifth frame.
Higgins roared back with a wonderful 140 break and followed it up with a 73 to get within one, the tide appearing set to turn.
Carter was ahead in the controversial eighth frame, but even so the incident on the yellow proved decisive as he took it before closing out the match.
“It was very tough," said Carter.
"A great match to be involved in, but it was hard out there. The balls went scrappy, there were lots of long frames, but I’m delighted to win.
"There is pride of performance, but I was pleased with the way I took the balls in the last frame, so I’m delighted."
Higgins, who made no mention of the incident himself, offered no excuses as he suffered a disappointing defeat to the player he'd labelled 'Denmark' - a reference to the Danes winning the 1992 European Championship as lucky losers.
“I was very poor tonight. I tried to blag it for a couple of frames by playing quickly. It was a poor game and it wasn’t enjoyable at all.
“I probably dragged Ali down. He was playing decent safety and keeping me tight. My safety was all over the place. It was reminiscent of my game with Yan Bingtao in the quarters of the UK Championship.”
Murphy dominates against Perry
Shaun Murphy was the first man into the semi-finals of the Dafabet Masters after a 6-3 victory over Joe Perry.
It's been a tournament packed with upsets at Alexandra Palace, but Perry never looked like following up his first-round defeat of Ding Junhui as the key moments all went the way of his opponent.
Murphy went in 3-1 up at the interval when it perhaps ought to have been 2-2, and then took a 5-2 lead after Perry missed a red which would have seen him cut the deficit back to just a single frame.
Instead, Murphy mopped up and went on to reach his first Masters semi-final since winning the title in 2015.
"I'm pleased to win. It was a big game for me, first chance to get in the semis in five years here," said Murphy.
"Obviously the most important stat is getting to six first, which I did, but there were times in the match I was really struggling.
"I thought Joe was so unlucky, so often, with splits and cannons and things. On another day I might not be sat here."
Murphy took the first frame, looking fluent among the balls as a series of small cannons unlocked the winning points.
Perry was in first in the second frame, opening up a 30-point lead before committing a foul when bridging over the cluster in attempt to keep his break going.
Murphy looked set to double his advantage, but a break of 60 came to an end with a missed double and Perry did well to take the remaining 35 points to level the scores.
The first sign of real weakness from Murphy came with a missed black off its spot in the third frame, bringing a 22 break to a premature end, and this was Perry's chance to lead with the balls nicely spread.
However, he managed only a point, failing to find position on a colour, and while laying a safety he'd missed a big opportunity. He was made to pay when Murphy took control of the exchange, before a 66 break put him back in front.
But it was frame four which really hurt Perry and in the end left him with too steep a hill to climb.
First, he missed black after potting a long opening red, then he scored only 14 points having been handed a second opening, he managed just six when handed a third, and then missed black off its spot again when within a couple of reds of levelling the scores.
This time there appeared to be an excuse - TV coverage revealed a very definite sound, perhaps a door closing, just as he withdrew the cue - but the result was that Perry's door to the semi-final was effectively slammed shut as Murphy established a 3-1 lead.
Upon the resumption it was again Perry who had the first chance, scoring 22 before breaking down in the balls. This time, he got lucky, Murphy suffering a kick as Perry came back to the table for his highest break of the match so far, 46 points just enough to get back within one.
The best snooker of the match came in the sixth frame, as Murphy produced a 120 clearance after Perry had gifted him an opening red with a careless safety. This was much more like it, a rare demonstration of cue-ball control as the Magician at last got the white ball under his spell.
There was a chance for another frame-winning score in the next but Murphy missed an easy red to middle, scoring just 21. Perry then had his, and while finally reaching a half-century, 50 wasn't enough. He needed one more red, missed it, and Murphy stole the frame.
What should have been 4-3 was instead 5-2, and while Perry kept his hopes alive briefly, he again was made to suffer for failing to be ruthless. Murphy, much the more fluent without being at his best, sealed victory with a brilliant pot with the rest, a delicate cannon, and a 32 break which ended Perry's bid to become the oldest ever winner of this historic event.
"Murphy potted the important balls, the pressure balls," mused Ronnie O'Sullivan in the Eurosport studio, and he was right. Perhaps, were he being honest, his main reaction to this first quarter-final is that he might have been better off playing. Murphy must surely perform better than this to win the title, and yet he's probably the man to beat.
"It's about winning, at the end of the day," added Murphy, who confessed to being frustrated at his inability to take practice-table form into the arena with him. "Sometimes your biggest opponent out there can be yourself."
And there's a lot of truth in that.
Watch the London Masters LIVE on Eurosport and Eurosport Player with analysis from Ronnie O'Sullivan, Jimmy White and Neal Foulds.
Tournament schedule and round-by-round results
- All matches up to and including semi-finals best of 11; final best of 19
ROUND ONE
Sunday 12 January
Afternoon Session (1300 - BBC/Eurosport)
- Ding Junhui 3-6 Joe Perry
Evening Session (1900 - BBC/Eurosport)
- Mark Selby 4-6 Ali Carter
Monday 13 January
Afternoon Session (1300 - BBC/Eurosport)
- Neil Robertson 5-6 Stephen Maguire
Evening Session (1900 - BBC/Eurosport)
- Mark Allen 1-6 David Gilbert
Tuesday 14 January
Afternoon Session (1300 - BBC/Eurosport)
- Judd Trump 3-6 Shaun Murphy
Evening Session (1900 - BBC/Eurosport)
- John Higgins 6-1 Barry Hawkins
Wednesday 15 January
Afternoon Session (1300 - BBC/Eurosport)
- Kyren Wilson 6-2 Jack Lisowski
Evening Session (1900 - BBC/Eurosport)
- Mark Williams 2-6 Stuart Bingham
QUARTER-FINALS
Thursday 16 January
Afternoon Session (1300 - BBC/Eurosport)
- Shaun Murphy 6-3 Joe Perry
Evening Session (1900 - BBC/Eurosport)
- Ali Carter 6-3 John Higgins
Friday 17 January
Afternoon Session (1300 - BBC/Eurosport)
- Stephen Maguire v David Gilbert
Evening Session (1900 - BBC/Eurosport)
- Kyren Wilson v Stuart Bingham
SEMI-FINALS
Saturday 18 January
Afternoon Session (1300 - BBC/Eurosport)
- Semi-final 1
Evening Session (1900 - BBC/Eurosport)
- Semi-final 2