Alvaro Quiros fired a flawless round of 64 to establish a four-shot lead at the halfway stage of the Dubai World Championship.
The big-hitting Spaniard started the day four off the lead after an opening 68 but was quickly into his stride with two birdies in the first four holes.
Another birdie at the eighth took him to the turn in 33, and he picked up three more shots before finishing in spectacular fashion, two huge strikes at the 620-yard closing hole setting up an eagle and a round of eight-under for the day.
"I hit a lot of good shots to the greens," said Quiros, a winner in Dubai back in February. "I've not been that precise from the tee to be honest.
"Yesterday I was hitting the driver and three-wood better than today, but the good score came today.
"There are some Spanish guys over there saying 'it's yours, you have it', but It's just the second round, we are just halfway, so it doesn't mean anything."
Having stood one ahead of McIlroy on the 18th tee, Quiros soon found himself four clear with the Northern Irishman failing to get up and down from a tricky spot back at the 17th.
And when McIlroy, chasing the victory he needs to have any chance of topping the Order of Merit, found trouble from the tee at the final hole he had to settle for a six, a disappointing round of 71, and suddenly five back.
Luke Donald put himself in a position to strike over the weekend with a round of 68 that saw him move up the leaderboard with a hat-trick of birdies to finish, his four under par total just three behind McIlroy.
It was a huge move for the world number one, who needs to finish ninth or better to clinch the Order of Merit even if McIlroy wins this week, as well as topping the PGA money list.
Approach
After parring the previous seven holes Donald suddenly sprang to life making 15-foot putts on the 16th and long 18th while holing from 10 feet in between.
"Just personally in terms of my mental approach, that was huge," he said. "It would be foolish of me to rely on him (McIlroy) not winning. I know what's at stake and it's big deal to me.
"I was frustrated out there. I tried to stay patient, but it's tough. I want to finish off what I came here to do and those last three holes brought a smile to my face.
"There's certainly a lot of pressure. I'm still making a few mistakes - maybe because of what's at stake. When there's a lot on the line you feel more nervous, but sometimes it's a good thing."
Overnight leader Peter Hanson, Paul Lawrie and Sergio Garcia were the most prominent players to lose ground on the rest of the field, the Swede rescuing his round with a birdie at the last to remain in the final group for round three.
Hanson failed to get anything going after his opening 64 but he hung in with successive birdies at 13 and 14 before getting up and down at the last to go round in level par.
Lawrie held a lead of three early in the day, picking up shots at the first two holes before nearly acing the fourth, but a bogey at the eighth was a portend of things to come and the Scot recorded three bogeys and a double coming home to slip back to six under par - he too finished on a high with a 15-foot birdie.
McIlroy will tee it up with Englishman Robert Rock on Saturday after adding a solid 69 to Thursday's 68 - matching five birdies with a couple of dropped shots.
Paul Casey made his move with a six-under-par 66 that included an eagle at the seventh, his round spoilt by a solitary bogey at the 17th leaving him in a tie for fifth with Lawrie.
The other round of 66 came from Sweden's Fredrik Andersson Hed, who was clearly irked by finishing his first round with four bogeys, starting as he did with a hat-trick of birdies with his only slip coming, like Casey, at the penultimate hole.
