Ronnie beats Ding Junhui

The four-time Masters champion settled the quicker of the two players at the tournament's new venue, Alexandra Palace, to take a 4-1 lead.

Ding hit back to win the next three frames, but O'Sullivan found some form and sealed the match with a 125 break.

O'Sullivan will face either Judd Trump or Stuart Bingham, who play on Monday.

"It was a tough, tough match to be fair," O'Sullivan told BBC Sport. "To lose a frame like [frame six] was tough because you think he [Ding] could punish you and you could lose three - and that's what happened.

"I had chances [in the next two frames] but my head went and it wasn't back until he levelled at 4-4. The pressure was on him then.

"I liked it [the new venue]. It's different to Wembley. It was a great crowd and a fantastic atmosphere."

The 36-year-old once again threatened to quit the game last December after losing to Trump in the second round of the UK Championship.

However, following his defeat of Ding in London, O'Sullivan, who has pulled out of a number of tournaments this season which has resulted in him dropping down the rankings and facing the prospect of having to qualify for the World Championships in May, insisted he has a future in the sport - but it will be on his terms.

"I'm not prepared to travel 28 weeks a year living out a suitcase anymore so I have decided that if I have to qualify, I have to qualify," said O'Sullivan who was watched by son Ronnie and daughter Lilly Jo.

"I do want to play snooker. I just want it on my terms. [My son and my daughter] are what's important so if I can get a bit of both, that's what I'm going to do."

O'Sullivan, who beat Ding 6-2 in the 2005 quarter-finals as well as his 10-3 demolition of the youngster in the 2007 final, surprisingly missed a black off its spot in the opening frame, allowing Ding a great chance to take it.

But the defending champion broke down on 58, and a brilliant long red saw O'Sullivan, who has dropped down to 16 in the world rankings, clear up with a break of 36.

Chinese star Ding, who beat Marco Fu 10-4 to win his maiden Masters title last year, was first in in the second with 21, but O'Sullivan responded with 57, which proved sufficient for a 2-0 lead.

O'Sullivan sunk another long red and made 48, but broke down when trying to split the reds. Ding, who was looking to reach the Masters final for the third time, hit back with breaks of 45 and 31 to get a score on the board.

O'Sullivan restored his two frame advantage with a beautifully constructed break of 76 to lead 3-1 going into the mid-session interval and then extended that lead with a break of 62.

In frame six, the momentum appeared to shift when Ding, who had come to the table 45-10 down after O'Sullivan broke down on 36, fluked a red which rattled the top left pocket's jaws before sliding across the top cushion and dropping in the top right pocket.

The Chinese player cleared up to win the frame and then mistakes from O'Sullivan, who appeared to suffer a lapse in concentration after losing the previous frame, in the next two allowed Ding to level the match at 4-4.

Ding, who arrived at Alexandra Palace having lost his last four matches against 'The Rocket', had the first chance in frame nine, but breaks of 51 and 20, which included a long red under pressure, allowed O'Sullivan to stop the rot.

Winning frame nine appeared to settle O'Sullivan as the world number 16 fired in a break of 125, the highest of the match, his 50th Masters century and the first ton to be compiled at Alexandra Palace, to progress to the last eight.