LLANELLI beat Prestatyn 1-0 in the Welsh Cup quarter-finals to set-up a last four meeting with The New Saints.
Andy Legg's well-struck free-kick in the 17th minute was all that separated the sides in a scrappy affair at Stebonheath Park.
The semi-final tie will be played on Saturday 9 April at a neutral venue to be announced.
There will be four meetings between TNS and Legg's men between now and the end of the season. As well as the cup semi, the sides are due to clash in the final of the Loosemores League Cup and both home and away in the Welsh Premier.
The Park Hall meeting between the sides was due to be on April 9, so will now have to be re-arranged.
Meanwhile, Bangor City failed to increase their lead at the top of the Welsh Premier as they were beaten 2-1 at home by Neath, who scored second-half goals through Kristian O'Leary and Lee Trundle.