Tiger Woods was set to do the same until his only bogey of the day on the 18th.
World number 267 Roberto Castro is taking three-shot lead into the second round. He has made just seven cuts in 14 events this season.
McIlroy, a two-time major winner, is without a win so far in 2013 after five victories around the world last year.
The 24-year-old was probably hoping for something solid on Thursday and duly reaped the benefits of a conservative approach after missing the cut in his previous three appearances in golf's 'unofficial fifth major' with a scoring average of 74.
Woods, who replaced McIlroy at the top of the rankings earlier this year, had never had a bogey-free round in 55 previous attempts in the tournament, but at least carded a five-under 67, his first sub-70 opening round in the event.
McIlroy told Sky Sports 2: "It's my best score around this course by a long way so I'm very happy. I kept my ball in play for the most part and didn't make any stupid mistakes.
"I made a couple of good par saves, but apart from that I was giving myself a lot of chances for birdie. If I can continue doing that over the next three days I should be there or thereabouts going into Sunday.
"You have to find the balance between being conservative but also aggressive as well. I adopted quite a conservative strategy off the tee this week because I feel my iron play has been so good.
"Even if I leave myself a bit further back I am still giving myself good chances for birdies."
Meanwhile, Dubliner Padraig Harrington and Sergio Garcia were five off the lead after rounds of 68, while Lee Westwood matched the 69 of Masters champion Adam Scott.