China\'s Ding Junhui opened up a 5-3 lead over world number two Mark Selby after the opening session of their world championship quarter-final on Tuesday. Selby opened with his eighth century of the tournament, a break of 124, but Sheffield-based Ding, playing in his first quarter-final at the worlds, capitalised on a series of errors from his opponent. He took the next four frames, helped by breaks of 58, 63 and 65. Selby hit back with an 87 to make it 4-2 only for Ding to respond with 77 to restore his three-frame advantage. A 74 break saw 2007 runner-up Selby stay in touch with the best-of-25 frame contest to conclude on Wednesday. Former champions John Higgins and Ronnie O\'Sullivan were level at 4-4 after their opening session. From 4-2 and 35-0 behind, O\'Sullivan conjured an 89 break and followed it with a 105 to level the tie. \"It\'s always a great occasion, wherever you play him,\" Higgins said of his opponent. \"He\'s got the Jimmy White and Alex Higgins factor, the crowd are on his side and that generates a buzz that you love playing in. \"Playing O\'Sullivan anywhere probably generates the same buzz as playing in a final against most other people.\" Higgins holds the edge in the number of ranking events the two players have won, 23 to O\'Sullivan\'s 22, after landing the UK Championship and Welsh Open titles since his return from a six-month suspension. O\'Sullivan pitched up in Sheffield having not won a ranking-event match since September, but his focus has returned. \"John\'s given everyone half a season start and still won more than everyone else,\" O\'Sullivan said. \"He\'s a class player, he\'s been the best player in the world for the last three or four years probably.\" Judd Trump, the 21-year-old qualifier who was rated an 80/1 outsider last month, was two frames away from a place in the semi-finals as he claimed an 11-5 lead over last year\'s runner-up Graeme Dott. Mark Williams, the 2000 and 2003 world champion, was also closing in on the last four, and a tie with Higgins or O\'Sullivan. The Welshman had led Mark Allen 5-3 after their first session but stretched his lead to 11-5, finishing with a break of 127.