O'Sullivan had been in superb form prior to meeting Robertson

Robertson won the first two frames against O'Sullivan and although his opponent made it 2-1, Robertson won the next three to claim victory.

Higgins, the defending champion, lost 5-2 to Mark King, while Australian Neil Robertson beat O'Sullivan 5-1.

The Scot never recovered from losing the first four frames to King.

Interview: Australia's Neil Robertson

"It went wrong all night. My concentration was all over the place from the first frame," he admitted. "That was the telling point in the match.

I kept my composure because I knew I was playing well and I knew I would be creating chances for myself Neil Robertson

"My safety was atrocious and I was leaving chances on a plate for him. You've got to perform, but I just didn't turn up.

"I was confident about my game and even at 4-0 still felt brilliant and that I could get back, but all night I struggled."

Stoke qualifier Jamie Cope sealed a semi-final spot with a 5-4 win over Joe Perry, clinching the decider 48-39.

"I just had to hold myself together," said Cope, who had earlier knocked in breaks of 50, 62 and 53.

"If I can carry on playing alright, I can win it."

Friday's final match saw Scot Alan McManus see off Englishman Ian McCulloch 5-3.

Robertson had lost all five of his previous ranking event quarter-finals but O'Sullivan admitted: "He was on fire. He just blew me away."

The world number three added: "You have to give him credit. He didn't miss a ball.

"Sometimes you come up against someone like that - he deserved his win. I wish I could have given him a better game, but I didn't.

"You can't just turn up with your B game and think you're going win, you can't.

"Today he played fantastic. If he carries on playing like that he'll fancy his chances of winning the tournament."

Robertson added: "I will definitely say this is the best performance of my career so far.

"I'm very happy with the way I played. I've scored heavier in matches and maybe played better, more explosive stuff. But playing like that against someone like Ronnie O'Sullivan is so much harder.

"I kept my composure because I knew I was playing well and I knew I would be creating chances for myself.

"It was a great result for me. I said before the match I was probably one of the few players who can beat him when he's playing well. I definitely believed that.