Ronnie O'Sullivan faces a knock-out blow at the Dafabet World Championship on Saturday unless he can belatedly find his best form.

The defending champion has been outshone by Joe Perry over their opening two sessions and O'Sullivan must overhaul a 9-7 deficit in the best-of-25-frame tussle to progress to the quarter-finals.

Nothing could separate the pair in Friday's afternoon session as they took four frames apiece to leave world number 15 Perry in pole position to cause the biggest upset of the tournament so far.

It could have been even better for the 39-year-old Cambridgeshire cueman had he not missed a host of chances to take the final frame of the session and head into Saturday four frames ahead.

O'Sullivan looked to have turned the corner in the match when he put together a break of 93 in Friday's opening frame but was soon on the back foot once more as Perry re-established his lead in the next frame and soon eased further clear.

A rare error from Perry allowed O'Sullivan to hit back with a break of 66 and, when he followed that up with success in a safety-filled 13th frame he was back in the match at 7-6.

That would normally open the floodgates for an O'Sullivan rally but it failed to materialise as Perry kept his nerve to record breaks of 86 and 73 in taking a three-frame lead.

Five-time champion O'Sullivan pinched the last frame of the session but it was clear he would need to improve to reel Perry in and reach the last eight of a tournament he remains - despite his overnight deficit against Perry - the hot favourite to win.

Mark Selby won a battle of former Crucible beaten finalists as he saw off an afternoon fightback from Ali Carter to record a 13-9 victory and progress into the last eight.

Two-time runner-up Carter fought back to take the final two frames on Friday morning to trail 9-7 heading into the evening session.

But he could not keep up the momentum when play resumed and Selby held on, setting up a clash with Alan McManus or Ken Doherty for a place in the semi-finals.

The tussle between that veteran pair will continue on Saturday. McManus headed Doherty 4-3 after a gruelling first session.

Scottish 43-year-old McManus compiled his first Crucible century in nine years - coincidentally the last was also against 44-year-old Dubliner Doherty - as he established a 4-1 lead.

Despite struggling to find any semblance of the form that brought him the 1997 world title, Doherty clawed his way back before the session was abridged with seven frames played.

Barry Hawkins and Ricky Walden resume on Saturday afternoon with the repeat of last year's semi-final once again providing drama.

Hawkins stormed out to a 3-0 lead but was pegged back by Walden who drew level at 4-4 at the close of the first session.

And it was Walden, who lost 17-14 to Hawkins 12 months ago, that stepped up the pace in the evening to ensure he heads into the deciding session 9-7 ahead.