Ronnie O'Sullivan built a 5-3 lead over Matthew Stevens and Ali Carter led Stephen Maguire by the same scoreline at the end of Thursday's opening session in the World Championship semifinals.

But three-time champion O'Sullivan's slender advantage owed something to an act of sportsmanship by his Welsh opponent who called a foul on himself in the second frame that neither the ‘Rocket' nor referee Brendan Moore had spotted. That helped O'Sullivan establish a 4-0 lead before Stevens battled back with a break of 80 in the eighth frame reducing the Englishman's lead to two frames.

Meanwhile, Carter put his struggles with Crohn's disease, a bowel condition he has had to manage for nine years, to one side. Both players struggled to find the form that had seen Carter knock out Judd Trump, last year's losing finalist, and Maguire beat seven-time World champion Stephen Hendry in the quarterfinals in what turned out to be his fellow Scot's final match before retirement.

The results: Semifinals (best of 33 frames): Ali Carter (Eng) leads Stephen Maguire (Sco) 5-3; Ronnie O'Sullivan (Eng) leads Matthew Stevens (Wal) 5-3.

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