Wales' Williams was 7-1 down overnight and at one point today was 12-5 behind, but a remarkable turnaround has seen him finish the evening only 13-11 back.

This included winning five consecutive frames and posting a 137 break, the largest of the semi-finals so far.

Englishman Trump is four frames away from a place in the final, but Williams has the momentum, including winning the last frame of the day.

Their semi-final is due to conclude tomorrow at Sheffield's Crucible Theatre. 

Ronnie O'Sullivan is 10-6 up versus John Higgins in the other semi-final.

England's O'Sullivan and Scot Higgins were tied at 4-4 overnight and then 6-6 at the mid-session intermission, but six-time world champion O'Sullivan then reeled off four frame wins in a row to take charge of the match.

All four semi-finalists are former winners for the first time in World Snooker Championship history.