Ronnie O'Sullivan suffered a disappointing first-round exit at the Northern Ireland Open as he lost the final three frames to lose 4-3 to world-ranked 110 player Ka Wai Cheung in Belfast.
After a strong start from Cheung, O'Sullivan won three frames in a row before a valiant comeback from the Hong Kong qualifier knocked out the 50-year-old.
For Cheung it was his first win in a main-draw knockout match at a ranking event and he will now face Stuart Bingham for a spot in the last 16.
O'Sullivan became the eighth top-16 player to lose of the nine who had played at the Waterfront Hall so far, with Judd Trump later adding to that tally as he went down 4-3 to fellow Englishman Martin O'Donnell.
O'Donnell led three times in the match but Trump responded on each occasion to draw level, before world number 56 O'Donnell prevailed in the deciding frame.
Trump compiled breaks of 91, 94 and 52, with O'Donnell putting together three breaks of 50 or over to seal his first win over the former world champion and world number three.
Mark Selby, Neil Robertson, Xiao Guodong, Shaun Murphy and Ali Carter fell on Sunday, with Chris Wakelin, Barry Hawkins, Trump and O'Sullivan following them out of the tournament on Monday.
O'Sullivan had not played at a tournament in the United Kingdom since April's World Championship 13-12 second-round loss loss to John Higgins and it was not the victorious return he would have hoped for.
The world number 13 was making a first appearance at the NI Open since 2022 and was playing in a Home Nations event for the first time since the 2024 English Open.
Cheung made a bright start and claimed the opening frame but O'Sullivan responded to that early setback by claiming the next three frames in speedy and emphatic fashion.
Breaks of 56,138 and 72 saw him move 3-1 ahead but that was as good as it got for the three-time NI Open finalist.
Just as it looked like O'Sullivan had found form, he missed a good chance on a red in the fifth and Cheung clinched the frame after a run of 88.
Cheung's confidence grew and he forced a deciding frame with another impressive 77.
Another costly miss from O'Sullivan gave the 27-year-old the chance to advance which he did with a break of 61.
"I'm very happy that I can still play well under pressure," reflected Cheung when he spoke to BBC Sport NI after the match.
"To be honest, as you can see in the arena, Ronnie had a lot of fans. I was actually scared a little bit before the match but this is the best experience.
"No matter if I had lost or won, I think it is the best experience for my snooker career and that is all I need.
"To play the legend of the game and to win, this is very satisfying for me."
In Monday's other afternoon games, Hawkins was beaten 4-2 by Daniel Wells but former world champion and 2024 NI Open champion Kyren Wilson progressed to round two with a comfortable 4-0 win over Ben Mertens.
Wakelin lost 4-1 to Welshman Jamie Jones earlier in the day, while Luca Brecel got the better of Lei Peifan by the same scoreline.
Scot John Higgins are among those in action during the evening session.