Defending champion Ronnie O'Sullivan admitted he felt "awful" after reaching the semi-finals of the English Open in controversial circumstances with victory over China's Luo Honghao.

O'Sullivan recovered from 3-2 down to win 5-3 at the K2 Leisure Centre in Crawley, but not without enjoying a vital stroke of luck when a foul he accidentally committed in the sixth frame went unnoticed.

Luo had said he was hoping to at least pot one ball against his idol and that was all the 18-year-old left-hander managed in the opening frame before missing a long blue. O'Sullivan then made the 957th century of his career with a clearance of 118.

A break of 90 took O'Sullivan 2-0 in front but Luo fought back superbly in his first ranking event quarter-final with breaks of 90, 67 and a total clearance of 136 - which he completed by knocking in the black right-handed - to take a 3-2 lead.

O'Sullivan drew level with a contribution of 73 in the next frame, but only after a foul early in the break went unnoticed, neither O'Sullivan or referee Tatiana Woollaston noticing the five-time world champion brushed a red with his cue after potting a different red with the rest.

Even after the match it took being shown several television replays during an interview with Eurosport before O'Sullivan realised what had happened.

"Oh no, oh s***," the 42-year-old said. "I didn't even see that in a million years. I'm happy to go play the match again if they want to. I feel awful after you showed me that."

Told that TV commentator Dave Hendon was the only one who initially noticed, O'Sullivan added: "I wish Dave Hendon would have said something and stopped the match, that's what they should do. I've done it in commentary before."

O'Sullivan won the seventh frame with a break of 82 after Luo had been first in with 56 and sealed victory in the next frame to set up a semi-final against Mark Davis, who had earlier beaten Ryan Day 5-1.

Davis, 46, has yet to reach the final of a ranking event but looked in superb form with breaks of 66, 95 and 92 helping him to a comfortable win over Welshman Day, who had to change a damaged tip during the interval.

The other semi-final will see Stephen Maguire take on former world champion Stuart Bingham after Bingham beat Ali Carter 5-2 with the aid of a break of 138 and Maguire overcame Noppon Saengkham 5-3.