Mark Allen and Wu Yize played out the longest frame in Crucible history during the second session of their World Championship semi-final.

A farcical 14th frame, in which eight reds were left covering the black over the bottom right corner pocket, lasted 100 minutes and 21 seconds.

It contributed to a shortened session that ended 7-7, featuring only six of the eight frames scheduled, and it delayed the resumption of the other last-four match between John Higgins and Shaun Murphy.

The impasse that left the match without a ball being potted for 55 minutes, and eventually prompted referee Marcel Eckardt to warn the players, was ended when Allen fouled and nudged the black in.

That allowed Wu to pick up his only frame of the afternoon, albeit another 30 more minutes elapsed before he was able to do so.

“In a nutshell that frame is an embarrassment to snooker, and the referees’ and the players’ association need to try to work out a way so that never happens again,” said six-time world champion and BBC pundit Steve Davis.

Up until that point it had been a complete reversal of fortunes from Thursday, when Wu held sway with a devastating display of potting that left Allen largely confined to his chair.

The shift of momentum began immediately, Allen wrapping up the opening frame of the day after Wu completely missed the final red – which he was attempting to snick into the left corner.