Ronnie O'Sullivan turned on the style just when he looked to be wobbling to stay ahead of Ali Carter in their Betfair World Championship second-round showdown.

A gripping middle session saw O'Sullivan struggle at times, especially with his long potting and safety play, but he bookended his afternoon in the Crucible with two breaks of 86 and finished 9-7 in front.

They return tomorrow night to finish off, with 13 frames the target for a place in the quarter-finals, and Carter must know he missed a great chance to at the very least level the match.

On Paul Hunter Day at the Crucible, set up to remember the much-missed three-time Masters champion and to promote the foundation set up in his name, O'Sullivan was almost victim of the kind of comeback he faced when the late Leeds potter won the third of his titles at Wembley in 2004.

Beginning the second session armed with a 5-3 lead, O'Sullivan scored heavily in spells but when Carter got to 7-7 it was the underdog who had all the momentum.

Hunter in his pomp might have capitalised but Carter could not.

O'Sullivan, who has beaten Carter in two Crucible finals, looked to be feeling the pressure of the occasion, but he regained the lead with a fluent 73 break.

And when Carter missed a long red in the final frame it gave the 37-year-old O'Sullivan another chance.

He never looked like missing on his way to restoring the two-frame cushion, with Carter left to stew over how he had been mostly the better player in the session and yet had been unable to make it count.

Before Carter produced his own wayward long red, it was O'Sullivan's potting that was looking shaky from anything beyond medium range.

At a tournament where burnout has been cited as a factor in the early demise of so many big names, O'Sullivan was supposed to be the freshest man out in the field, having taken most of the year off since securing his fourth title at the Crucible last May.

He looked that way when he fired in 105 to lead 7-5.

But O'Sullivan missed a frame-ball black to grind to a halt on 61 in the next frame and Carter cleared with 63.

It was soon 7-7, but that was it for Carter.

O'Sullivan made a pair of brilliant cross-table doubles that had the crowd purring.

It means O'Sullivan remains on course to stay unbeaten against Carter at major tournaments. This is their 13th meeting and tonight looked like it was going the same way as the previous 12.

O'Sullivan is also on target to defend the world title, a feat nobody has achieved since Stephen Hendry won back-to-back from 1992 to 1996.

The overriding sense inside the Crucible is that a fifth world title for O'Sullivan is on the way.

Shaun Murphy, a quarter-finalist this year and champion in 2005, said: "I've had a really consistent year but when Ronnie's in the event it's hard to look past Ronnie."

And Ken Doherty, the 1997 world champion, believes O'Sullivan has a crucial advantage as his rivals feel the toll of a long season.

"The likes of Mark Selby, Neil Robertson and Mark Allen have played almost 100 matches this season. That's quite a lot," Doherty said.

Those big names have fallen but O'Sullivan remains on the prowl.

Doherty told BBC Radio 5 Live: "He's come into this tournament so fresh and he's a fit young man as well. That may play a small part in the whole of this World Championship."

A host of low-ranked players remain in the tournament, with 21-year-old world number 40 Michael White through to the quarter-finals in the bottom half of the draw, along with Barry Hawkins.

Doherty likened the situation to the 1986 championship when long shot Joe Johnson triumphed.

"The bottom half is so open that we maybe could have a 'Joe Johnson year' this year, with someone coming completely from the field, a complete outsider, getting easily to the final at least," Doherty said.

Ding Junhui had breaks of 59, 98, 74, 81 and a closing 103 in surging from 6-2 behind against Mark King to lead 9-7, with that match ending tomorrow afternoon.

Ricky Walden stayed ahead of Robert Milkins in their all-English tussle, finishing the day 10-6 in front after getting the better of their exchanges over morning and evening sessions, pursuing the incentive of a quarter-final against qualifier White.

World number eight Stuart Bingham was involved in a tight battle with 40-year-old Hastings man Mark Davis.

Davis, who has reached three ranking event semi-finals this season to earn a top-16 ranking for the first time, trailed 7-5 at one stage this evening before reeling off three in a row. But Bingham ended positively with breaks of 44 and 43 to level up at 8-8, ahead of tomorrow afternoon's concluding session. They are vying to face O'Sullivan or Carter.