No.8 seed Auger-Aliassime stunned Stefanos Tsitsipas on Friday, with the Greek subsequently lauding the 18-year-old as "the most difficult opponent" he has ever faced, but he came undone against Lopez, with the Spaniard coming out on top 6-7 (3-7) 6-3 6-4.
The Canadian edged a tight first set on a tie-break but he fell 2-0 down in the second after dropping his first service game of the week after 44 consecutive holds.
That was all Lopez needed to level the match and he further strengthened his grip by breaking to lead 3-2 in the decider.
Auger-Aliassime saved four match points at 3-5 but Lopez got it done in the next game to move into his third Queen's final.
Lopez will face Simon, who beat Daniil Medvedev 6-7 (4-7) 6-4 6-3 in the other semi-final.
A thrilling contest was settled when the Russian, troubled by a lower back injury, double-faulted to hand Simon a 3-5, third-set lead which he duly converted.
"It was hard to recover for this one. Daniil and I practise together and we knew it was going to be tough," Simon said.
"It's hard for me to hit through him, but when you play a match and you're a bit tired, sometimes you relax and play great. I was feeling the ball great, even if I wasn’t feeling great in the legs."