Blackpool’s James Cahill is five frames away from producing one of the all-time snooker shocks at the Crucible.
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Cahill, the first amateur to play at the World Championship, leads Ronnie O’Sullivan 5-4 at the halfway stage of their first-round match in Sheffield.
World number one O’Sullivan looked out of sorts throughout Monday’s session and Marton 23-year-old Cahill rarely looked in awe of his fancied opponent.
O’Sullivan is chasing a sixth world crown and the delicately poised tie resumes on Tuesday morning.
Debutant Cahill, who won through the qualifying competition in Sheffield last week, settled immediately to win the first frame.
He fell behind 2-1, 3-2 and 4-3, but came back on each occasion to level before taking the final frame of the session.
O’Sullivan had breaks of 51 and 86 to win frames two and three but Cahill kept his composure with a 71 to go into the mid-session interval all square.
O’Sullivan looked rattled from that point and more errors crept into his performance against Cahill, who will rejoin the professional ranks next season.
Six-time world champion Steve Davis told the BBC: “It was probably the worst session of snooker Ronnie O’Sullivan has played this season.
“I just think he is playing too quickly. It’s as if he wanted to get out of there. It didn’t look like he was enjoying being there.
“James Cahill’s attitude has been brilliant. He has played with rhythm and confidence.”