McIlroy was satisfied with an opening 71 which left him just three shots off the lead held by American Bill Haas and two adrift of defending champion Adam Scott, Bubba Watson and Louis Oosthuizen.
But the 24-year-old was anticipating a tough course to get even tougher by the time he resumed his bid for a third major title in the final group of the day at 1359 local time.
"It was a good day at the office, anything in red figures was a good effort out there," said McIlroy, whose best finish in five previous Masters is a tie for 15th in 2011, when he led by four going into the final round but collapsed to a closing 80.
"When I saw the pin positions in the morning I thought they didn't want people to get too many under par.
"The set up was much more difficult than it has been in the past. Going out Friday afternoon the greens are going to be firmer. I wouldn't mind if they were a little softer. They're fast already. By Sunday they're going to be pretty dicey."
Watson carded the only bogey-free round of the day to finish three under alongside Oosthuizen, the man he beat in a play-off to win the green jacket in 2012.
But Scott's form was equally ominous, the Australian matching his opening 69 from 12 months ago despite a double-bogey five on the 12th, where he dumped his tee shot into Rae's Creek in front of the green.
The 33-year-old Australian is looking to join Jack Nicklaus, Nick Faldo and Tiger Woods in winning back-to-back titles, but said: "I haven't really (thought about it). It's been mentioned and it gets mentioned to every guy who is coming back to defend I'm sure.
"It's an amazing group of guys, great champions, but it's so much golf to play yet, I can't even think about it. My goal teeing off was to play three good days and get myself in contention and give myself a chance like I did last year."
Seven players shared fifth place on two under par, with another eight on one under including 54-year-old former champion Fred Couples, 50-year-old Spaniard Miguel Angel Jimenez, 20-year-old duo Matteo Manassero and Jordan Spieth and Scotland's Stephen Gallacher.
Gallacher held outright second place after a birdie on the ninth took him out in 33, and although he bogeyed the next three holes, a birdie on the 15th gave the 39-year-old an excellent 71 on his tournament debut.
"I'm a wee bit disappointed," said Gallacher, who was advised by putting coach Dave Stockton to treat it as the North Georgia Open. "It's a special place, one in a million place. It's just hard to get that out of the way and just concentrate on the golf.
"At the end of the day it's a golf tournament, so you've got to be trying to play the course."
Luke Donald admitted he only had himself to blame for a "pretty dumb mistake" which left him with a mountain to climb to make the halfway cut at the 78th Masters on Friday.
Donald's opening 79 at Augusta included a two-shot penalty for touching the sand with his club after failing to escape from a greenside bunker at the first attempt on the par-four ninth.
"Pretty dumb mistake," the former world number one wrote on Twitter. "I can accept the odd poor shot but making a mistake like that is pretty frustrating and so unlike me."
Donald was far from the only big name to run up a big score however, with the last three major winners a combined 16 over par thanks to an 80 from US PGA champion Jason Dufner and matching rounds of 76 from playing partners Justin Rose and Phil Mickelson.
Three-time winner Mickelson had two sevens on his card to equal his worst opening round in 22 Masters appearances, while US Open champion Rose at least rallied from being six over par after 12, a hole where he was one of many players to find water off the tee on the 155-yard par three.
"It wasn't the best day for me," Mickelson said. "There were possibilities for low scores and I actually played well. I know it's hard to say that, but I actually played well and I just made a lot of little mental errors.
"I made a lot of little mistakes around the green, but I didn't hit anything unsolid. I just misjudged the shots."
Dufner was level par after nine holes but came home in 44 with a quadruple-bogey nine on the 13th.
Jonas Blixt was Europe's highest-placed player after a 70 on his Masters debut, with Miguel Angel Jimenez enjoying a two-shot lead on four under before a bogey on the 11th and double-bogey on 12 resulted in a 71. Lee Westwood bogeyed the last for a 73, Sergio Garcia and Darren Clarke both shot 74 and Ian Poulter a 76.
Notable figures from the opening day's play:
0 - The number of times Bill Haas had broken 70 in 16 previous rounds at Augusta before an opening 68 gave the 31-year-old American the lead.
2 - Penalty shots picked up by Luke Donald for touching the ground in a greenside bunker on the ninth after failing to escape at the first attempt. Donald went on to shoot 79.
3 - Jack Nicklaus, Nick Faldo and Tiger Woods are the only three men to have won back-to-back titles. Defending champion Adam Scott was one behind Haas after a 69.
9 - US PGA champion Jason Dufner's score on the par-five 13th.
16 - Combined total over par of the last three major champions - Dufner (+8), Justin Rose (+4), Phil Mickelson (+4).
13 - Masters titles won by honorary starters Arnold Palmer (four), Jack Nicklaus (six) and Gary Player (three).
24 - Number of Masters debutants in the field.
12 - Number of shots Kevin Stadler took less than his father Craig, the 1982 champion.
19 - Age of English amateur Matt Fitzpatrick, the youngest player in the field.
4.47 - The average score on the par-four 11th, making it the hardest hole on the course.
3.42 - The average score on the 155-yard 12th hole, which played the second hardest.
84 - Worst score of the day, recorded by South African Branden Grace courtesy of six bogeys, two double bogeys, one triple bogey and a solitary birdie on the sixth.