Luca Brecel's bid to end Ronnie O’Sullivan’s 17-month reign as world number one was halted by Ding Junhui at the Cazoo British Open.
The Belgian, who beat O’Sullivan on his way to winning a first World Championship at the Crucible earlier this year, needed to reach the third round in Cheltenham to climb to the top of the rankings.
But Brecel, 28, slipped to a 4-3 defeat in the opening round against China’s Ding.
O’Sullivan announced last week that he would miss the tournament owing to medical reasons.
Also on day one, Belfast's Mark Allen knocked in breaks of 81 and 94 as he saw off Scotland's Anthony McGill. Allen lost the first frame, before winning four on the bounce to progress.
Judd Trump came from 2-0 down to secure a 4-2 win over Ukrainian Anton Kazakov. Defending champion Ryan Day was a comfortable 4-0 winner against Sean O’Sullivan.
There were further wins for Mark Selby, Neil Robertson, Shaun Murphy, Mark Williams, and Barry Hawkins, who beat 17-year-old debutant Stan Moody 4-0.
On Tuesday Dublin veterans Fergal O'Brien and Ken Doherty are in action.
O'Brien is up first against Thailand's Noppon Saengkham before Doherty takes on Matthew Stevens, the man who beat him in the famous Masters Final in 2000.