McIlroy marked his return following a six-week break in ominous fashion for the rest of the field at the European Tour's season-ending event with a six-under round that included six birdies.
That was enough to leave the world number one and Race to Dubai winner one shot clear at the top of leaderboard alongside Shane Lowry at the end of play on Thursday.
"Nice first day back at the office..." posted McIlroy on Twitter.
The Northern Irishman, bidding to win the prestigious Dubai tournament for a second time, added to reporters: "You never expect to start like that.
"But I've been hitting the ball well for the last couple of weeks that I've been practicing and it was just a matter of trying to take that good range play on to the course.
"And I was able to do that, which I'm really happy about."
Lowry, who picked up shots on four of his last six holes on the back nine, showed great mental resolve to respond from his final-round slump at the Turkish Open last weekend.
The 27-year-old was in title contention at the start of Sunday's play before dropping five shots in two holes.
Lowry said: "Anytime you shoot 66 anywhere it's quite decent but this golf course can be difficult at times.
"I played one really bad hole but other than that I played beautiful golf. So yeah, I'm pretty happy."