Mark Selby, who had dreaded coming to the world championship because of his poor form, produced a shock result to win his first title here, beating the greatest player of them all, Ronnie O'Sullivan, 18-14.

He had won three Masters titles and the 2012 UK Championship but this was the trophy that Selby, aged 30, coveted more than any other. At 8-3 and then 10-5 down on Sunday his victory looked highly unlikely. But on Monday he won the first four frames and when he sealed his emotional victory he had taken 13 out of 17, including the last two on Sunday night. It was the best final for years. O'Sullivan, going for a hat-trick here, had crushed Barry Hawkins and Ali Carter in the previous two events.

In a tearful post-match interview Selby said: "It can't get any better than this. It was amazing. There's no better way than beating Ronnie in the final. I dug in, I did not play pretty snooker early on but as it wore on I thought I did well. It was a tough day yesterday and the last two games were huge. To get back into 10-7 gave me a chance. 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."

The defending champion, O'Sullivan, was generous in defeat. He said: "I want to congratulate Mark on a fantastic tournament. He's been the best player over 17 days, that was tough he had me in all sorts of trouble. In the end I was numb as he was too strong and tough. Maybe I would have thrown in the towel a few years ago but I tried my hardest. He was too good.

"Everything I did he had an answer for I can't have it all my own way all the time. I've had some great victories here but losing is part of the sport. I will have to go away and lick my wounds and come back and go one better next time."

This victory means that Selby is once again the No 1 ranked player in the world – he is also one of only nine to have won the "triple crown" of world, UK and Masters titles. But his only previous victory this season was in the Antwerp Open European Tour event in Belgium – where he came from 3-1 down to beat O'Sullivan in the final. He had also come back to beat O'Sullivan 9-8 in the 2008 Welsh Open, having been 5-8 down.

Selby was not at his best here until the closing frames of the match. He struggled to win frames at a single visit and his first and only century, 127, did not arrive until the 30th frame. But he did play immaculate matchplay snooker, with precise safety and an unyielding tactical acumen to make life as difficult as possible for Sullivan – and that is the only way to beat this most gifted of players.

This was O'Sullivan's first defeat in six final appearances here and, despite his magnaminity, it represented a desperate disappointment as he attempted to draw level with Steve Davis and Ray Reardon with half a dozen titles.

Steve Peters, the sports psychiatrist who has been advising O'Sullivan during championships, looked in need of a lie-down in a darkened room as Selby staged his astonishing fightback. There are some players who never know when they're beaten. Think Graeme Dott, think Peter Ebdon and think, too, Mark Selby. O'Sullivan doesn't like coming up against opponents such as these, and Selby just might top the list.

Selby is a little like the Black Knight from Monty Python and the Holy Grail, who wanted to fight on after losing both arms – "Just a flesh wound," he declared.

On Sunday Selby had looked exhausted and out of touch after his 12-hour marathon of a semi-final against Neil Robertson. On Monday he looked rested and reinvigorated. And as he fought back there were nods of approval from The Grinder himself, Cliff Thorburn, the champion of 1980, whose grimly unremitting style was the despair of a different generation.

"Mark is the modern-day Thorburn," O'Sullivan added afterwards. "He's got the game to win many more if he wants to."

They call Selby the Jester from Leicester, but that is not always the case. In the crucial 21st frame, in which he edged ahead for the first time in the match, 11-10, a member of the audience coughed during the player's back-swing and received a look that might have turned her to stone.

When O'Sullivan won the next frame to level at 11-11, at 4.30 in the afternoon, it was the first frame he had won since 10 o'clock the previous evening. But he was in dire need of the end of session interval when he missed a simple pink to go behind 12-11.

There should have been eight frames in the afternoon but the going was so tough that only six were played, with the players returning to play, potentially, 12 more in the evening.

O'Sullivan returned for the final session wearing a new, slicker hairstyle. And it appeared to work as he knocked in a seven-minute century to level the match at 12-12. But he won only two more. There were few people in Sheffield who thought Selby could catch up with O'Sullivan, and most of them were related to the popular Leicestershire player.