Ronnie O'Sullivan was eliminated from the Northern Ireland Open on Monday night as he conceded the deciding frame to Ka Wai Cheung.
The match ended 4-3 after the Hong Kong qualifier took control following a valiant effort by the world number 13.
O'Sullivan had taken a 3-1 lead early in the contest but could not maintain his advantage against the rising player.
Cheung claimed the opening frame before O'Sullivan responded with three consecutive victories in quick succession.
A missed opportunity on a red ball in the fifth frame allowed Cheung to extend his lead and force a decider.
The world number 13 secured the final frame after a break of 88, ending his campaign at the tournament.
This result represents O'Sullivan's second consecutive first-round exit from a ranking event in recent months.
He had not competed in a tournament held within the United Kingdom since his second-round defeat to John Higgins in April.
The match marked O'Sullivan's first participation in the Northern Ireland Open since 2022 and his debut in a Home Nations competition since the English Open last year.
Cheung, ranked 110th in the world, recorded his first victory in a main-draw knockout match at a ranking event.
He will now advance to face Stuart Bingham in the last 16 of the tournament.
O'Sullivan became the eighth top-16 player to lose from the nine competitors who had reached the Waterfront Hall so far.
Judd Trump also exited on Monday, joining Chris Wakelin, Barry Hawkins and others who failed to progress.
Mark Selby, Neil Robertson, Xiao Guodong, Shaun Murphy and Ali Carter were among those eliminated on Sunday.
The tournament continues with further matches scheduled at the venue in Derry City.
Cheung's confidence grew throughout the contest as he forced a deciding frame with another impressive break of 77.