McIlroy and playing partner Viktor Hovland carded matching third rounds of 66 on the Old Course to finish 16 under par and open up a four-shot lead over the chasing pack led by Cameron Smith and Cameron Young.
“We fed off each other really well,” said McIlroy, who holed out from a bunker for an eagle on the 10th and also made five birdies. “We both put ourselves into a great position going into tomorrow.
“I know I am going to need one more day like the last three days to get the job done.”
McIlroy lifted the Claret Jug in 2014 and won the US PGA Championship a month later, but injury prevented him from defending his title at St Andrews in 2015 and he has endured a long wait for more major success.
“It would mean everything because of what I’ve been through the last few years trying to get that fifth one,” the world number two added.
“But here we are talking about not getting ahead of myself…I need to go out and keep the same mindset I’ve had the last three days.
“I know if I play my game I can shoot scores like this on this course and I need to do it one more day and then I’ll give you all the superlatives and everything else if that does happen.
“I’m a better player than I was 12 months ago. I have a lot of belief in myself. I’ve done it before, I know I can do it again. I’ve just got to go out and stay in my own little world, shoot a good score and hopefully that’s enough.”
Speaking about the bunker shot on the 10th, McIlroy added: “That was special. Sometimes to win these sort of championships you need a bit of luck here and there – that was part skill, part luck.
“It was a bonus, to make an eagle there was huge. Every part of my game has felt good and I just need to keep it going for one more day.”
While McIlroy is seeking a fifth major title, Hovland is chasing his first and the first for any male Norwegian player.
“I don’t think there’s any other place that would top it,” the 24-year-old said of winning at St Andrews.
“Growing up in Norway I always watched The Open Championship for way longer than I ever did, for example, the Masters. To win a major that’s closest to home, that would be really cool.
“It’s pretty crazy from where I grew up and so far away from playing the PGA Tour, European Tour, for that matter major championships.
“Just to be here is very special, but to have a chance to win one, yeah, I have to pinch myself, but that doesn’t mean I’m going to hold back tomorrow.”