World Snooker Championship
Date: 18 April-4 May Venue: Crucible Theatre, Sheffield
Coverage: Live coverage each day on BBC Two, BBC Red Button and BBC Sport website (UK only), updates on BBC Radio 5 Live.
Higgins hit the highest break in a scrappy opening to the semi-final
Scotland's John Higgins emerged with a 6-2 lead from an error-strewn first session of his World Championship semi-final with Mark Allen at the Crucible.
Both players were guilty of making basic mistakes in a nervy opening, but Higgins knocked in a 55 in frame three.
The balls were blamed and a decision to change them was made at the mid-session interval with Higgins leading 3-1.
Northern Ireland's Allen pinched the fifth but Higgins found some momentum, knocking in a 114 and 75 to lead 6-2.
Allen, who knocked Ronnie O'Sullivan out in the second round, took an untidy first frame and although Higgins responded with breaks of 40, 55 and 49, neither played with any fluency.
During the mid-session interval it is believed that concerns were raised over the state of the baize, which was re-clothed overnight and playing slowly, and the balls themselves after both players were troubled by kicks.
The baize was ironed and the balls changed and Allen, bidding to become the first man from Northern Ireland to reach the final since Dennis Taylor's victory in 1985, took the fifth with a break of 41.
But Higgins, who is aiming to win a third world title, rattled in the only century to re-establish his two-frame lead before edging a tight seventh and closing the session with a well-crafted 75.
England's Shaun Murphy, who won the title in 2005, starts his semi-final against Neil Robertson at 1900 BST.