The seven-time world champion was stunned by world No 110 Cheung Ka Wai, who rattled off three frames in a row to clinch a 4-3 win – his first in a main-draw knockout match at a ranking event.
O’Sullivan was playing his first match in the UK since his 13-12 second-round defeat to John Higgins at the World Championship in Sheffield in April.
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But despite racking up a 138 total clearance in the second frame, a series of blunders let in the Hong Kong qualifier to nervelessly take advantage and seal his place in the second round.
Playing in only his third tournament of the season, O’Sullivan, 50, lost the opening frame, but showed few signs of rust as he hit back to take the next three with breaks of 56, 138 and 72.
Cheung, who joined the tour in 2024 and had only previously won main-draw matches in the Championship League, looked down and out when O’Sullivan got in first in the fifth, but left a red dangling over the bottom pocket.
Cheung responded with a superb break of 88 and levelled the match after he pounced on a simple missed brown by O’Sullivan to summon a break of 77.
O’Sullivan was in first again in the decider, but missed a blue to the middle and his inexperienced opponent carved out a match-winning break of 60 to heap more woe on his ailing opponent.
