(first to 18 wins the title)
Five-time champion O'Sullivan dramatically missed a routine pink which would
have sealed the last frame of the afternoon session.
Earlier Selby, who trailed 10-7 overnight, won the day's first four frames, taking his winning run to six.
O'Sullivan faces a real fight when the best-of-35 match resumes at 19:00 BST.
Eight frames were scheduled for the day's first of two sessions, but only six were played as the players fought out a gripping battle.
Leicester potter Selby, seeking his first world title, looked refreshed and claimed five of the six.
O'Sullivan, who sought advice from his psychologist Steve Peters during the interval, is behind going into the last session for the first time in six final appearances at Sheffield's Crucible Theatre.
Selby showed none of the fatigue that appeared to set in early in the final, with breaks of 55, 52 and 74 - his highest of the match so far - to level up at 10-10.
He then took the lead for the first time in the match, punching the air, after winning a topsy-turvy 21st frame just before the mid-session interval.
O'Sullivan returned after the break slightly more relaxed with a 50 run to make it 11-11 despite an error-strewn frame.
But Selby won a thrilling 23rd frame lasting 50 minutes as O'Sullivan missed a sitter on the pink.
Frame 18: 32 to open for Ronnie, a real stalemate with yellow & green over baulk pockets, Ronnie goes in-off, Mark in with a chance, Mark doing a really good job with a tough table, 55 break leaves Ronnie needing a snooker, Mark takes the frame well deserved. 10-8.
Frame 19: Ronnie snookers Mark behind the green off the break, eventually 22 foul points later, Mark hits a red, Mark keeping Ronnie at arms length small break reds all around the pink & black. Good safety from Mark puts Ronnie behind the green, foul called twice, Mark with a break of 38 at the moment, The Jester as he is known looking far more relaxed & alert after his epic semi final win v Neil Robertson, a 52 break courtesy of a missed frame ball red, Ronnie can tie, missed black, should be Mark's frame now. 10-9.
Frame 20: Selby with the early advantage, Ronnie had a early chance missed red, Mark finding it hard going, a great break of 74 to level at 10-10.
Frame 21: Ronnie with a slight lead in a tactical frame, 28 break for Mark, leads by 27 points, 34 lead, 35 on, colours left Mark clears the lot. 11-10.
Frame 22: Mark first in, 36 break confidence is a wonderful thing, missed red Ronnie in. 50 break (14 ahead), a further 8 points, clear tension both missing easy balls, eventually Ronnie pots the green, we're level at 11-11.
Frame 23: 8 point lead for Selby in a tight frame, Ronnie at the table, a 28 break for Ronnie 2 point lead for Selby, Mark snookered behind the black, missed three times, yellow now safe, Ronnie mis-hits a safety, Mark snookers Ronnie, hit by the Rocket, roles reversed Ronnie in trouble, this is the last frame of the session due to the players needing a hour between sessions. Colours left, Mark pots the yellow, missed green, Mark pots the green safety on the brown, 1 point lead for Ronnie. Blue knocked safe for now, brown left over the pocket for Ronnie, blue for Ronnie, missed pink incredible! Black for 12-11 Selby, IT IS THERE! See u tonight!
Mark Selby
leads defending champion Ronnie O'Sullivan 12-11 going into Monday night's final
session of the World Snooker Championship final.
Earlier Selby, who trailed 10-7 overnight, won the day's first four frames, taking his winning run to six.
O'Sullivan faces a real fight when the best-of-35 match resumes at 19:00 BST.
Eight frames were scheduled for the day's first of two sessions, but only six were played as the players fought out a gripping battle.
Leicester potter Selby, seeking his first world title, looked refreshed and claimed five of the six.
O'Sullivan, who sought advice from his psychologist Steve Peters during the interval, is behind going into the last session for the first time in six final appearances at Sheffield's Crucible Theatre.
Selby showed none of the fatigue that appeared to set in early in the final, with breaks of 55, 52 and 74 - his highest of the match so far - to level up at 10-10.
He then took the lead for the first time in the match, punching the air, after winning a topsy-turvy 21st frame just before the mid-session interval.
O'Sullivan returned after the break slightly more relaxed with a 50 run to make it 11-11 despite an error-strewn frame.
But Selby won a thrilling 23rd frame lasting 50 minutes as O'Sullivan missed a sitter on the pink.
Mark Selby 18-14 Ronnie O'Sullivan
Frame 24: Ronnie straight in with a frame winning century 100 exactly 12-12.
Frame 25: Just two foul shots by Ronnie, 12 points given away, Mark with a nice red & brown down to the business end of the table, 34 break leads by 46 points. Only 24 to respond from Ronnie. A missed frame ball again for Mark. Safety battle with two reds left, Selby left a pot got it! Selby leads 13-12.
Frame 26: 14-12 Selby (39-64)
Frame 27: 15-12 Selby (12-90, Selby 56 break)
Frame 28: 15-13 Selby (79-0)
Frame 29: 15-14 Selby (66-47)
Frame 30: 16-14 Selby (0-131, Selby 127 break)
Frame 31: 17-14 Selby (14-87, Selby 87 break)
Frame 32: Ronnie leads in the frame by 29 points, but Mark Selby is the champion! He wins 18-14!
Leicester's Selby, runner-up
in 2007, was 10-5 behind but won 10 of the next 12 frames for a 15-12 lead.
Five-time champion O'Sullivan, who had never lost a world final, reduced the deficit to 15-14, only for Selby to win the next three frames.
His victory means he returns to the top of the world rankings and leaves O'Sullivan two behind Stephen Hendry's record tally of seven world titles.
Selby, 30, is £300,000 richer for his Crucible triumph and is the ninth player to win the Triple Crown, having won three Masters titles and a UK Championship.
He immediately dedicated his success to father David who passed away in 1999.
"It's not really sunk in but my father passed away when I was 16 and I always said I wanted to win the world title for him," he said.
"I dug in; I did not play pretty snooker early on but as it wore on I did well. There's no better way than beating Ronnie in the final."
Selby looked jaded on Sunday following his epic semi-final against Neil Robertson but managed to win the final two frames to stay within three of O'Sullivan, who wrapped up his last-four encounter against Barry Hawkins with a session to spare.
Selby has a history of overcoming deficits against O'Sullivan in big games. In the Welsh Open final in 2008 he came back from 5-8 down to win 9-8, and in the Masters final in 2010 overturned a 6-9 deficit to win 10-9.
Selby was a different player in Monday's first session, winning five of the six frames played and dragging his rival into unfamiliar territory, O'Sullivan having never before trailed heading into the final session of a Crucible final.
Selby made breaks of 55, 52 and 74 - his highest of the match so far - to level the contest at 10-10 before winning a tense 21st frame to lead for the first time.
O'Sullivan retook the lead after the mid-session interval, winning an error-strewn frame courtesy of a run of 50. But a jaw-dropping moment in the final frame of the session, which lasted 50 minutes, handed Selby the lead again.
The momentum having swung this way and that, O'Sullivan only needed a simple pink into the middle pocket to clinch the frame but inexplicably decided to play it with pace and missed the pot. Selby duly cleared up before punching the air and ramming his cue into the ground in a mixture of relief and elation.
Eight frames were scheduled for the session but it was a measure of the attritional nature of the action that they could only fit six in before the interval.
O'Sullivan won the first frame after the restart with a fluent break of 100, his third century of the match, to level at 12-12. But Selby won a 28-minute frame to restore his advantage before opening up a two-frame lead for the first time.
Selby compiled a break of 56 to increase his lead to three frames before the mid-session interval, meaning O'Sullivan had lost 10 of the previous 12 frames.
O'Sullivan came out firing after the restart, rattling in breaks of 30 and 49 to reduce the gap to two frames. And O'Sullivan potted a magnificent yellow down the cushion before clearing up the rest of the balls to make it 15-14.
But Selby reacted in style, knocking in a break of 127, his first century of the match, before compiling a run of 87 to get to within one frame of victory.
O'Sullivan had chances in the next frame to stave off defeat but after he missed the final red Selby pounced to complete a nerve-jangling clearance.
Mark Selby
staged a remarkable comeback to beat defending champion Ronnie O'Sullivan 18-14
in the final of the World Championship in Sheffield.
Five-time champion O'Sullivan, who had never lost a world final, reduced the deficit to 15-14, only for Selby to win the next three frames.
His victory means he returns to the top of the world rankings and leaves O'Sullivan two behind Stephen Hendry's record tally of seven world titles.
Selby, 30, is £300,000 richer for his Crucible triumph and is the ninth player to win the Triple Crown, having won three Masters titles and a UK Championship.
He immediately dedicated his success to father David who passed away in 1999.
"It's not really sunk in but my father passed away when I was 16 and I always said I wanted to win the world title for him," he said.
"I dug in; I did not play pretty snooker early on but as it wore on I did well. There's no better way than beating Ronnie in the final."
Selby looked jaded on Sunday following his epic semi-final against Neil Robertson but managed to win the final two frames to stay within three of O'Sullivan, who wrapped up his last-four encounter against Barry Hawkins with a session to spare.
Selby has a history of overcoming deficits against O'Sullivan in big games. In the Welsh Open final in 2008 he came back from 5-8 down to win 9-8, and in the Masters final in 2010 overturned a 6-9 deficit to win 10-9.
Selby was a different player in Monday's first session, winning five of the six frames played and dragging his rival into unfamiliar territory, O'Sullivan having never before trailed heading into the final session of a Crucible final.
Selby made breaks of 55, 52 and 74 - his highest of the match so far - to level the contest at 10-10 before winning a tense 21st frame to lead for the first time.
O'Sullivan retook the lead after the mid-session interval, winning an error-strewn frame courtesy of a run of 50. But a jaw-dropping moment in the final frame of the session, which lasted 50 minutes, handed Selby the lead again.
The momentum having swung this way and that, O'Sullivan only needed a simple pink into the middle pocket to clinch the frame but inexplicably decided to play it with pace and missed the pot. Selby duly cleared up before punching the air and ramming his cue into the ground in a mixture of relief and elation.
Eight frames were scheduled for the session but it was a measure of the attritional nature of the action that they could only fit six in before the interval.
O'Sullivan won the first frame after the restart with a fluent break of 100, his third century of the match, to level at 12-12. But Selby won a 28-minute frame to restore his advantage before opening up a two-frame lead for the first time.
Selby compiled a break of 56 to increase his lead to three frames before the mid-session interval, meaning O'Sullivan had lost 10 of the previous 12 frames.
O'Sullivan came out firing after the restart, rattling in breaks of 30 and 49 to reduce the gap to two frames. And O'Sullivan potted a magnificent yellow down the cushion before clearing up the rest of the balls to make it 15-14.
But Selby reacted in style, knocking in a break of 127, his first century of the match, before compiling a run of 87 to get to within one frame of victory.
O'Sullivan had chances in the next frame to stave off defeat but after he missed the final red Selby pounced to complete a nerve-jangling clearance.
