O'Sullivan had been one frame from victory

Robertson was 4-3 down in the best-of-nine match but then hit a 124 break to force a deciding frame, which the Australian won 74-45 for a 5-4 win.

He will now meet veteran Steve Davis, who beat Anthony Hamilton 5-4.

Andrew Higginson knocked in a maximum 147 break as he overwhelmed Ali Carter 5-1 and now plays Stephen Maguire, who upset in-form Shaun Murphy 5-3.

Maguire was a happy man after beating former world champion Murphy in Friday's final match.

I'm disappointed, but it's only one match Ronnie O'Sullivan

"I try hard to beat everyone but I think it would hurt even more than usual if Shaun had beaten me," said Maguire.

"We've always had a rivalry. I dislike him and I think he dislikes me."

Higginson's was only the third 147 in the tournament's 15-year-history.

The 29-year-old, who has earned less than £70,000 in prize-money during his 12-year career, will take home £20,000 for the second-frame break.

"I've made dozens of 147s in practice but never in a competition of any sort. It's quite a stage to make your first," said Higginson.

"After the last black, I was shaking like a leaf."

He added: "When I got to 56 the balls started to get a little awkward. I couldn't quite seem to land where I wanted but I kept potting them and getting myself out of trouble."

It was a roller-coaster of emotions for me Steve Davis

Higginson remained in a rich vein of form, putting together further breaks of 83 and 48 in scoring 342 points without reply on the way to leading 4-0.

Carter averted the whitewash by winning frame five, but Higginson closed the match out with his seventh century of the tournament.

After his defeat a phlegmatic O'Sullivan said: "I'm disappointed, but it's only one match.

"Here, I've been working on a few things to broaden the boundaries of my game. I'll do the same at the China Open - then it's off to Sheffield for the one that really matters.

"I've got no complaints. I've gone from really struggling to feeling half in control of my game."

Robertson said: "To be an elite player like the O'Sullivans and the Hendrys you've got to consistently beat the top guys. That's why I'm so thrilled by this.

"I was really confident. I don't know why, but Ronnie doesn't scare me as much as some other players."

Six-time world champion Davis recovered from 3-0 and 4-3 down to edge out Hamilton.

"It was a roller-coaster of emotions for me and given how it went I can't really believe I managed to snatch the verdict," said the 46-year-old.

Victory gave him a place in his 58th world ranking semi-final but the Essex player has not won a ranking event since his 1995 victory in this tournament.