Ali Carter extended his semi-final advantage over Stephen Maguire by finishing the second session 10-6 ahead at the Crucible.
Carter resumed Friday's play with a 5-3 lead but soon squandered that as Maguire capitalised on some poor safety shots to level the scores.
The 2008 finalist responded by winning the next two frames, leaving the Scotsman banging the table in frustration in what was a tense session of snooker.
Maguire regained his composure to record a break of 142 and pull a frame back, but Carter then won three on the bounce to take a four-frame lead.
Play resumes at 7pm on Friday, with the winner playing Matthew Stevens or Ronnie O'Sullivan, who leads the Welshman 11-5 in their last-four match.
