Defending champion Humphries was in fine form, averaging more than 100 and dropping just three legs all night.
Joyce averaged well himself by clocking up 94.42, but it wasn't enough as the world number one churned out a ruthless display.
"Ryan was brilliant but I am capable of these big performances," Humphries told Sky Sports afterwards.
"I believe in myself in these big moments and I had to put in that big performance. It felt special. I said to him he played well but I was just on fire.
"If I could win tomorrow it would complete the most incredible 12 months," he added.
"Five-nil is quite flattering as Joyce played well. I was relaxed tonight and I played my best."
Humphries is looking to add a second World Grand Prix title to his collection and continue a fine year which has yielded wins at the World Championship, World Matchplay and the World Cup of Darts for England.
The 29-year-old will have to get past unseeded Belgian De Decker in Sunday's final after he continued his storybook run with a 5-2 win over compatriot Dimitri Van den Bergh.
The other semi-final was a much more scrappy affair with both players averaging in the 80s, but it was enough for De Decker to reach a maiden major final.
"I'm over the moon – really, really happy," said De Decker.
"In the beginning of the game I was really nervous. There's been so much attention on this game and when I took the second set I relaxed, knowing that I can still play.
"I'm loving the feeling of winning, the crowd have given me so much support and I'm really looking forward to tomorrow."
