Ronnie O'Sullivan kept his hopes of a winning sixth world title alive after hauling himself back from the brink in the penultimate session of his semi-final against Mark Selby at the Crucible.

O'Sullivan looked to have lost focus with a series of poor safety play and rash shot choices, and seemed certain to fall five frames behind before rallying to finish just two frames adrift at 13-11.

Selby had started the morning with a hard-earned 9-7 advantage and his tough match-play tactics put him firmly in control against O'Sullivan, who often cut a frustrated figure in his chair.

Showing further signs of a career revival after two years of relatively sparse success, Selby immediately extended his lead to 10-7 despite narrowly failing to record the first century of the match when he fell short on 97.

O'Sullivan took advantage of a missed red by Selby in the following frame to reduce the deficit with a break of 45, before Selby restored his three-frame lead.

A potentially pivotal moment in the match came in the next frame where Selby looked set to go further ahead only to miss a straight brown and let in O'Sullivan to keep in touch at 11-9 at the mid-session interval.

But the warning signs started flickering for O'Sullivan in the next frame when a missed red to the middle let in Selby to increase his lead to 12-9 with a break of 72.

And when a rash red resulted in O'Sullivan accidentally potting the black in the 22nd frame, Selby duly knocked in his second consecutive 72 break to stretch his lead to four frames.

An increasing number of errors from O'Sullivan suggested he was in danger of sliding out of contention but a fluked snooker behind the brown enabled him to take advantage and clear from the last red to reduce the deficit again to 13-10.

And in a hugely important final frame of the session, O'Sullivan once again survived a succession of unlikely shot choices to leave it with all to play for at 13-11 later on Friday.