As a result, the former is now facing up to a long two days in Shanghai in the no-cut event.
McIlroy is now way beind after an error-strewn five-over 77 in the second round which took place on Friday.
The 29-year-old had actually reached the turn in 35 after three birdies and two bogeys.
But he bogeyed his 10th which was the first and he also triple bogeyed the next.
And that was aftermath of twice finding water and double-bogeyed 16.
Finau’s 67 left him three above English pair, Tommy Fleetwood (68) and Justin Rose (67) including overnight leader, Patrick Reed (72).
McIlroy’s problems at his 11th, the second, came after he had hit a perfect drive.
And that left him with only a short iron into the par five.
However, the world number five’s second shot flew over the green into a water hazard.
And in the wake of having mis-hit his chip, his ball trickled into the same hazard again.
The former world number one is in a share of 58th in the 77-man event.
And that is the case going into the final two days of the no-cut tournament.
Fitzpatrick three shots behind Reed after WGC-HSBC Champions first round
Englishman, Matthew Fitzpatrick is right now three shots behind leader, Patrick Reed after the first round of the WGC-HSBC Champions.
Fitzpatrick who also happens to be the world number 40 made eight birdies in his round of 67.
And after which he finished on five under par with compatriot, Tommy Fleetwood one stroke further adrift in Shanghai, China.
Masters champion, Reed carded a bogey-free eight-under-par 64.
And that was just good enough for him to lead by two from fellow Americans, Xander Schauffele and Tony Finau.
“It feels great,” Reed who was competing for the first time since the Ryder Cup said to reporters in China.
“To be able to come out here and to be the first event back and to feel like I hit the ball really solid.
“I had full control and that especially was with how windy it was today.
“It felt really good because if you can go out and shoot rounds like that in these kind of conditions, perfect.
“You know you are going to have confidence when the wind dies down and there are perfect conditions out there.” He suggested.