Putter woes stall Scott at Open
Australia's Adam Scott plays out of a bunker on the second hole during his second round of the British Open Golf Championship at Royal Liverpool Golf Course in Hoylake, north west England on July 18, 2014 - by Peter Muhly
The world number one from Australia started the day two adrift of the Irishman, who blazed a six under 66 on Thursday to take the lead.
Scott parred the opening hole but then twice missed with short range putts at the next two holes to fall back to two under, four off the pace.
The 2013 Masters champion, who could have matched McIlroy on Thursday were it not for a disappointing finish is wary of his rival's ability to destroy the field.
"You've got to keep pace with a guy like Rory," the Australian said.
"He has the potential to really put his foot down. I've seen him shoot lots of low rounds and there's nothing stopping him from doing that tomorrow either.
"I don't want him running away. We've seen that happen. Wins majors by eight."
McIlroy 's task for the day was to avoid the kind of "freaky" Friday flop that has bedevilled him all year.
The world number seven has made it a habit this year of going low in the opening round before slumping badly on the Friday, most recently at last week's Scottish Open where he shot a course record 64 only to take a 78 the next day.
But this is the first time he has led after the first round of a major since the 2011 US Open. On that occasion he powered away from the field to win his first major title by eight shots.
All the signs were green for 4/1 tournament favourite McIlroy at Royal Liverpool on Friday with his mid-afternoon tee-time set to get the best of the day's weather.
His 66, his second best round at an Open Championship, gave him an overnight one stroke lead over Italy's Matteo Manassero with seven players on four under 68.
These included Italian brothers Francesco and Edoardo Molinari, Americans Brooks Koepka and Jim Furyk, Sergio Garcia of Spain, Ireland's Shane Lowry and the dangerous Scott.
Fit-again Tiger Woods was one of nine players on three under after five birdies down the back nine provided the surest sign yet that he can still compete at the top level.
For Woods, playing in just his second tournament since back surgery in late March, the challenge is firstly to make it through to the weekend, something he did not achieve at Congressional last month in his return to action.
There were clear signs on Thursday that the confidence was surging back into his game and he was understandaly upbeat after his round.
"I felt good about a lot of things that I did out there today," he said.
"Especially coming back after that start I had (two bogeys in first two holes), to fight myself back into the championship. I feel pretty good about it."
Of the other big names out early on Friday US Open champion Martin Kaymer made the most headway getting to two under with three birdies in six holes.
Defending champion Phil Mickelson stayed at two over after four holes, top English hope Justin Rose stayed at even par after three and Masters champion Bubba Watson picked up two strokes to get to two over after five.