Mark Allen was beaten 4-1 by Scott Donaldson in the Northern Ireland Open while defending champion Judd Trump booked his place in the quarter-finals.
Antrim player Allen struggled in Milton Keynes against his Scottish opponent, who registered a highest break of 86 in the third-round march.
World number one Trump, going for a third straight NI Open title, came from behind to beat Luca Brecel 4-3 having been one frame from defeat at 3-2, before comfortably beating Martin Gould in the last 16.
Trump, who hit a maximum 147 in Wednesday's win over Gao Yang, will take on Donaldson in the quarter-finals after the Scot beat Noppon Saengkham following his win over Allen.
Last year's runner-up, world champion Ronnie O'Sullivan, defeated Matthew Stevens 4-2 on Thursday afternoon before overcoming Thepchaiya Un-Nooh by the same scoreline to book his place in the last eight.
Allen was among the favourites to win his home event as a player in form following his Champion of Champions title success.
But the former Masters champion could only manage a top break of 61, which helped bring him level at 1-1, in a disappointing display.
"I just never got started and lost the white a few times when I had chances," said Allen.
"Scott played much better that I did and deserved to win. He completely outplayed me. My game is good - I just need to work on a couple of things and I'm not too down on myself because of one bad day at the office."
Kyren Wilson, the world number four, is also out after a 4-2 defeat by Michael Holt.
World Snooker has been running all of its tournaments in Milton Keynes because of the Covid-19 pandemic.
The Marshall Arena has on-site accommodation, meaning everyone involved in the tournament can be contained at the complex.
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