Ronnie O\'Sullivan\'s bid for a fourth world snooker championship title gained momentum in a Crucible masterclass which gave the Englishman a 11-5 lead over semi-final opponent Matthew Stevens Friday. O\'Sullivan had built a 5-3 lead over the Welshman after Thursday\'s session, during which Stevens called a foul on himself which neither O\'Sullivan nor the referee had noticed. The \'Rocket\' took up where he left off, reeling off six straight frames and treating a rapt audience to breaks of 61, 62, 110, 98, 90 and 113 as he continued to show the form which suggests a final place is well within his grasp. Stevens, who had paid tribute to O\'Sullivan after the Englishman disposed of former world champions Peter Ebdon, Mark Williams and Neil Robertson in earlier rounds, needs to change tack dramatically if he is to avoid another 6-2 session that would seal O\'Sullivan\'s place in the final. Earlier, Stephen Maguire\'s frustrations spilled over and his hopes of reaching a first World Championship final took a major hit. Opponent Ali Carter, the 2008 runner-up, showed the temperament required to win a semi-final as he turned a 5-3 lead into a more comfortable 11-7 cushion. In theory, Carter was able to wrap up victory tonight as they returned for the third of four sessions. Maguire, 31, twice lost his cool before the mid-session interval, firstly with a wild attempt to escape from a snooker and then when he thumped the table to draw gasps from spectators. He later appeared to question the judgement of referee Leo Scullion, his fellow Glaswegian, when a foul was called.