Martin Gould awaits Trump in Saturday's final four in Llandudno.
But Williams felt luck played a major part in Englishman Trump's progress.
"It was a good game and could have gone either way," said two-time world champion Williams. "Towards the end, obviously he only won it through the luck he had."
Williams picked out two incidents on the blue ball as crucial - an escape and a pot by Trump.
"I had him snookered on the one and he somehow got away with it, and the blue he took on - he'd probably pot one of them in 1,000.
"And how I left the blue on the middle hole in the last frame is unbelievable.
"But I've had a fair bit of run myself over the last few months so I'm not going to worry about that."
Five-time world champion Ronnie O'Sullivan beat Graeme Dott 4-1 to set up his last-four encounter with Stuart Bingham, who beat Mark Davis 4-0 in the last eight.
"We both played poorly. I felt nervous for some reason and couldn't see the wood for the trees," said O'Sullivan.
"It was strange, one of those evenings and it was lucky Graeme played as badly as me.
"I missed so many balls, it was diabolical."
Saturday's semi-final draw:
Stuart Bingham v Ronnie O'Sullivan
Martin Gould v Judd Trump
Judd Trump reached the World Grand Prix semi-finals by potting the last black in the final frame to beat Wales' Mark Williams 4-3.