Barry Hawkins stung Mark Williams by producing a priceless big break under pressure to stay ahead in their gripping Betfred World Championship semi-final.
Williams trailed 9-7 overnight and the 43-year-old Welshman looked like pulling it back to 12-12 in their best-of-33-frame contest.
However, the two-time world champion, who is seeking to reach the Crucible final for the first time since he landed the 2003 title, instead ended the session 13-11 behind after a costly miss.
Williams led 50-0 in the final frame of the session but rattling the black around the corner pocket jaws presented Hawkins with a half-chance, and he capitalised with a 73 break.
That was a body blow to Williams but Hawkins was good value for his lead, having made breaks of 128 and 133 earlier in the session. Williams responded with 103 and looked the man more likely to take the heavily tactical frames, but he needed to raise his game in the evening session with the winning line coming into view for his opponent.
The second century from Hawkins was the 75th of this year's tournament and meant the sponsors handed over a cheque of £25,000 to the Jessie May children's hospice charity.
Speaking at the presentation of the cheque, Kent cueman Hawkins said: "I'm happy with that session. It was a big clearance there in that last frame which keeps my nose in front but there's a long way to go still."
Williams already boasts a unique set of world titles, having won not only at the Crucible earlier in his career but also at the Six-Red World Championship and the World Seniors.
Twelve months ago he was considering retirement after failing to qualify for the World Championship but has since earned more than £500,000 in a remarkable career resurgence.
He and Hawkins are vying for the right to face John Higgins in the two-day final, starting on Sunday. Four-time world champion Higgins carried a 13-11 lead into Saturday afternoon's concluding session against Wilson and went on to wrap up a 17-13 victory.