RONNIE O'Sullivan put an uncertain start behind him to share the opening eight frames of his 888.com World Championship semi-final with Stephen Hendry at the Crucible Theatre.

O'Sullivan won the final three frames of the initial session to level the best-of-33 showdown at 4-4 against seven-time champion Hendry.

O'Sullivan started as an odds on favourite to beat Hendry, whom he had thrashed 17-4 in their last Crucible meeting, a semi-final clash four years ago.

But his long potting has been his Achilles heel during the tournament with only a 57 per cent success rate and he tried to rein in his natural attacking instincts and play in a more conservative vein until the closing stages of the session.

Hendry took full advantage in the opening frame when O'Sullivan refused a long red and produced a poor safety shot.

Hendry, making a record 12th last four appearance, cued smoothly and fluently as he put together a superb 140 total clearance for the 718th century of his career.

It was fully half an hour before O'Sullivan potted his first ball of the match midway through the second frame but the world number one made too many unforced errors.

He looked set to nick the frame after knocking in the final long red but missed a thin cut on the blue and Hendry fortuitously doubled the same ball to leave his opponent needing snookers.

A break of 60 helped Hendry stretch his led to 3-0 and O'Sullivan was in danger of a pre-interval whitewash as he missed the kind of pots he would normally convert.

Both players were guilty of mistakes in the final frame before the break.

But O'Sullivan finally opened his account after Hendry failed to leave the white behind the black and stuck the final yellow over the middle pocket.