The 22-year-old, who went top of the rankings by holding off Tiger Woods to win the Honda Classic last week, went to the turn in a one-over-par 37 at the WGC-Cadillac Championship and already found himself a staggering eight strokes behind Adam Scott.
The Australian, who has one World Golf Championships title to his name already, covered the back nine in a brilliant 32 strokes and started the outward half eagle-birdie to go two clear of American Steve Stricker.
Most of the 74-strong field found the Doral course tough going – and bringing up the rear on four over were English pair Ian Poulter and Lee Westwood. Poulter was perhaps no surprise a week after being diagnosed with pneumonia, but Westwood came fourth last Sunday and finished with a 63 that was his best-ever round in America.
As for Woods, he picked up where he left off – with an eagle – but by the fifth was back to level-par, while Luke Donald, who has a chance to reclaim the No 1 spot this week, had three birdies in his first seven holes, but bogeyed the 17th and then double-bogeyed the next after pulling his second shot into the lake.
Scott, winner of the Bridgestone Invitational last August, birdied the 12th, 14th, 16th and 17th before hitting his approach to four feet on the long first and then converting a nine-foot chance at the next.
McIlroy, paired with Donald and Westwood in a clash of the sport’s top three, failed to get up and down from off the 11th green, sank an 11-foot birdie putt on the next, but could not save at par at the 18th after his drive finished close to the water.
Westwood’s problems began instantly when he thinned his greenside bunker shot to the 10th into the water. He took six there, three-putted the next and bogeyed the long 12th as well.
Spain’s Sergio Garcia, playing with Woods and defending champion Nick Watney, had four birdies and a bogey in his first six holes and was sharing third place with Dane Thomas Bjorn, Australian Greg Chalmers and Americans Charles Howell and Jason Dufner.
Scots Martin Laird and Paul Lawrie were one under after seven and five holes respectively, while Open champion Darren Clarke, England’s Justin Rose and Simon Dyson and Phil Mickelson were level par.
Abu Dhabi winner Robert Rock, who could secure a Masters debut with a high finish this weekend, was two over alongside Graeme McDowell, and one further back, despite two opening birdies, came Paul Casey in his first tournament since dislocating his shoulder snowboarding on Christmas Eve.
Scott failed to get up and down from sand on the sixth and was caught by Stricker, while Garcia was only one behind after a superb outward 31.
McIlroy just missed for eagle on the first, but then bogeyed the third and fourth – and it might have been worse.
The US Open champion’s second to the third hit the rocks on the edge of the lake, but bounced left back on to grass.
Donald, though, hit his approach to four feet there and with a second birdie in three holes was back to two under.