The Englishman got off to a flying start, breaking Clayton with a 15-darter and then producing a 12-darter and two 13-darters into a 6-0 lead. It wasn't until the seventh leg that Clayton got on the scoreboard with a 74 finish.
The Welshman then got a bit better in the match but it was far too late to bring the tension back. Clayton came back to 9-4, before Cullen decided the game with a 13-dart leg.
Later this evening Cullen will play in the final against the winner of the second semi-final between Michael van Gerwen en James Wade.