Humphries admitted he is not happy with how he is playing after also failing to score a maximum and averaging just 89.47 in his first-round win.
“I think the last two months it has been hard work,” the Englishman said on ITV X.
“I am out of sorts, out of form, and struggling to find something. Since [winning] the Premier League final I have not been in the form I would have liked.
“It is depressing because I feel like something is there but it’s not coming out.
“It’s really tough to not perform at the level you can. I am finding myself not playing very well at the moment – but I can bounce back.
“I don’t think I have had one good game of darts in two months. Everyone knows that and everyone can see that.
“That doesn’t stop me coming back tomorrow and being the world number one that everyone knows.”
Humphries beat Damon Heta in the final to triumph last year and they will face each other again after the Australian thrashed Dean Hyland 6-1.
Top seed Stephen Bunting, aiming to add to his victories at the Bahrain Darts Masters and Nordic Darts Masters earlier this year, progressed past Ben Robb 6-3.
Welshman Gerwyn Price missed double 12 for a nine-darter and averaged more than 103 as he edged out Jonny Tata 6-4 to set up a quarter-final against Northern Ireland’s Josh Rock.
