From 4-2 and 35-0 behind, O'Sullivan conjured an exquisite 89 break and followed it with a rapid 105 in which referee Leo Scullion was almost chasing him around the table to respot the colours.

The official from Glasgow twice almost tripped over television cameras while the crowd marvelled at the way O'Sullivan finished the session, which ended 4-4. They resume tomorrow afternoon and have an evening finish.

Higgins never had any doubt the contest with O'Sullivan would be an experience to savour.

"It's always a great occasion, wherever you play him," Higgins said.

"He's got the Jimmy White and Alex Higgins factor, the crowd are on his side and that generates a buzz that you love playing in.

"Playing O'Sullivan anywhere probably generates the same buzz as playing in a final against most other people."

It was appropriate they should finish on level terms, seeing as the parallels between the pair are impossible to ignore.

Both men are 35 years old, have three world titles behind them and entered this contest level on 104 Crucible centuries. Make that 105 now, after O'Sullivan's closing ton and Higgins' total clearance of 121 in the sixth frame.

Higgins holds the edge in the number of ranking events they have won, 23 to O'Sullivan's 22, after landing the UK Championship and Welsh Open titles since his return from a six-month suspension.

O'Sullivan pitched up in Sheffield having not won a ranking-event match since September, but his focus has returned.

His admiration of Higgins has never gone away.

"John's given everyone half a season start and still won more than everyone else," O'Sullivan said.

"He's a class player, he's been the best player in the world for the last three or four years probably."

Higgins will know he should be leading heading into tomorrow.

On a break of 35 in frame seven, he missed a straightforward black off its spot and sharp punishment followed.

Fifteen years have passed since O'Sullivan and Higgins first clashed at the Crucible. It finished 13-12 in the quarter-finals that time, and it would hardly be a shock if another deciding frame was required tomorrow.

There have been more meetings in Sheffield, most notably in the 2001 final when O'Sullivan landed his first world title, taking that match 18-14.

Higgins has been far from amazed to see O'Sullivan, almost with the flick of a switch, hit form in Sheffield after a mostly dismal season.

The Scot said: "Of course not. He's so talented that he can do that."

O'Sullivan is seeing early results from working with sports psychiatrist Dr Steve Peters, and after seeing off Shaun Murphy in the second round, he has high hopes of staying around for the duration.

Mark Selby was favourite for the title as he headed into his quarter-final against China's Ding Junhui, who had come from 12-9 behind to beat Stuart Bingham 13-12 in the second round.

But Selby faces a scrap tomorrow after Ding opened a 5-3 lead over the Leicester potter.

Selby had a tournament record of six centuries in his last-16 win against Stephen Hendry and began with another today against Ding, 124.

However Ding then strung together four frames in a row, featuring breaks of 58, 53, 63 and 65.

Selby made it 4-2 with runs of 51 and 87 but Ding guaranteed himself an overnight lead with 77 in frame seven. Selby then finished off with 74.

Judd Trump, the 21-year-old qualifier who was rated an 80/1 outsider as recently as last month, was two frames away from a place in the semi-finals as he claimed an 11-5 lead over last year's runner-up Graeme Dott.

There was a tantrum from Scottish potter Dott tonight in the 13th frame, at 10-2 down and with his game in bits, when he played a poor positional effort and lashed out in anger on his next shot, barely connecting with the white ball.

He lost that frame, but won the next, and then remarkably went close to a 147.

It would have been only the 10th maximum in Crucible history, but after potting all 15 reds and adding the blacks, he missed the yellow and the break halted on 120.

Dott, though, was suddenly playing well and ended the session with another century, 122, to give himself a glimmer of hope for tomorrow afternoon's final session.

Bristolian Trump, though, looked in good shape to set up a meeting with the winner of the match between Selby and Ding.

Mark Williams, the 2000 and 2003 world champion, was also closing in on the last four, and a tie with Higgins or O'Sullivan.

The Welshman had led Mark Allen 5-3 after their first session but stretched his lead over the Antrim cueman this evening to 11-5, finishing with a break of 127.

They play to a finish tomorrow morning.