Ronnie O'Sullivan was second best to Joe Perry in the opening session of their World Championship second round match at the Crucible.

The 38-year-old five-time world champion trailed 4-1, before reducing the deficit to 5-3 in this best-of-25 frame clash.

Perry was quick out of the blocks, taking the opening frame. O'Sullivan levelled but his Wisbech opponent took the next two frames to go in at the mid-session interval 3-1 up.

O'Sullivan was in trouble when he lost the fifth frame, however he produced a 116 clearance to reduce the deficit to two frames.

The defending champion appeared to be finding fluency when he followed up with a decisive 75 to move to within a single frame of Perry.

However, a break of 99 from Perry checked the progress of 'The Rocket' and gave the 39-year-old the edge for when the match resumes on Friday afternoon.

The other second round match started on Thursday saw Mark Selby establish a lead over Ali Carter, with the margin again 5-3.

Two-time beaten finalist Carter took the opener with a clinical 102 break, before Selby responded with a 38 and 84 in the second, then took the third after emerging from a safety battle.

Selby won the next two frames and with a 5-1 advantage looked set for a healthy lead.

However, Carter dug in with a 44 after Selby had spurned the first chance and then, after Selby failed to make the most of a fluke, rattled in his second century of the afternoon.

Earlier, 2010 champion Neil Robertson posted the tournament's highest break as he wrapped up a 10-2 victory in his first round match against Robbie Williams.

The Australian left-hander and world number one, who held a 7-2 overnight lead, did not take long to wrap up victory, following a clearance of 140 with a knock of 102 as he sealed a second round match against Mark Allen.

Dominic Dale secured a first win at the Crucible since 2000 in the remaining first round tie, turning an overnight 7-2 advantage over Mark Davis into a 10-5 success. He will face Michael Wasley, ranked 73 in the world, in the second round.