O'Sullivan is hoping to win the tournament for a record seventh time and the former world number one ignored the after-effect of an illness to win the last four frames at London's Alexandra Palace.

'Rocket' Ronnie will play Hong Kong's Marco Fu in the semi-final on Friday.

Fu, a Masters runner-up in 2010, eased through 6-2 against Mark Allen which included the highest break of the tournament so far of 140.

Fu showed no nerves when Allen pulled back from 3-0 down to 3-2 as he rattled off the next three to book his place in the last four.

Robertson, who has featured in three of the last five Masters finals, started strongly and won the first frame with a break of 74 but O'Sullivan responded with a 63 to level the contest.

O'Sullivan went 2-1 up with a 51 but Robertson levelled and then knocked in breaks of 59 and 62 to win the fifth.

However, the Australian's resistance was broken by O'Sullivan, who won the next four frames to secure an impressive fightback.

"Physically I feel better. The last three or four days I've been really ill and this is the first day I actually feel better," O'Sullivan said.

"I've needed a bit of luck to get through. They're all tough matches but I just keep hanging in there. I felt I just dragged him down but that's all you can do sometimes.

"Maybe you miss easy balls as you get older and that's the difference between winning and losing but hopefully I've got three decent years in me."