SHANGHAI, October 31, 2013 (AFP) - The 24-year-old was helped by world number three Phil Mickelson blowing up in disastrous fashion on the long eighth hole, the 17th of his round, as the American found the Sheshan Golf Club water twice en route to a quadruple-bogey nine.
Moments earlier McIlroy had carded his eighth birdie of the day on the same hole to move to seven under. Mickelson then bogeyed his last hole, the ninth, to drop five shots in two holes and plummet from outright second to tied 28th in the 78-man field with a 71.
On Wednesday McIlroy had said he was capable of winning the tournament with the way the course was set up.
On Thursday he proved it was no idle boast as his swagger and, more importantly, his short game returned to leave a field containing 40 of the world's top 50 in his wake.
The last time McIlroy had even held a share of the lead was after the first round of the Wells Fargo Open at Quail Hollow on May 2.
He leads by two from Spain's Gonzalo Fernandez-Castano, who carried on his fine form from last week when he won the BMW Masters across the city, by carding a five-under round of 67.
Also on 67 was Jamie Donaldson of Wales.
Back on four-under are the English pair Justin Rose and Tommy Fleetwood. They were joined on that mark by two Americans, Bubba Watson and US PGA Tour Rookie of the Year Jordan Spieth.
But it was McIlroy's day. The revival he had threatened, with a second place in the Korea Open a fortnight ago and a solid performance -- tee to green at least -- last week at Lake Malaren, at last materialised.
His confidence buoyed by an exhibition match victory over Tiger Woods on Monday in Hainan, McIlroy got off to the best possible start on his opening hole, the 10th.
Having just seen playing partner and US PGA champion Jason Dufner hole his second shot with a sand wedge for an outrageous eagle two, McIlroy almost matched it with a lob wedge of his own -- leaving a tap-in for the first of seven birdies in his opening 11 holes.
His only blemish of a near flawless card -- McIlroy afterwards modestly gave his own ball striking just seven out of 10 -- came at the 11th when the Northern Irishman was plugged after driving into a fairway bunker.
He was forced to advance the ball just 20 yards but still almost saved par with a seven-iron to 15 feet. However, the putt just slipped by.
He got the shot back straightaway at the 225-yard par three 12th when he rolled in a 20-footer for birdie and never looked back.
Birdies followed at 14, 16 and 18 to get to four-under at the turn, and then McIlroy hit what he described as one of his "shots of the day" on the first.
A rare errant drive found deep rough on the right but he struck a nine-iron imperiously from 160 yards to 12 feet and holed the putt.
Another birdie on the long par-five next hole, where he was just short of the green after a driver and a three-iron, took him to six under, until a final birdie at the eighth gave him daylight on the field.
"It was very good," a smiling McIlroy told AFP straight after his round. "And it was needed."
McIlroy this time last year was world and European number one.
He now lies 62nd in the Race to Dubai standings and must record a good finish this week to guarantee being among the top 60 who qualify for the season-ending DP World Tour Championship in Dubai in two weeks' time. McIlroy is defending champion there.
Leading scores on Thursday after the first round of the $8.5 million World Golf Championship-HSBC Champions at Sheshan Golf Club, Shanghai (par 72):
65 - Rory McIlroy (NIR).
67 - Gonzalo Fernandez-Castano (ESP), Jamie Donaldson (WAL).
68 - Jordan Spieth (USA), Bubba Watson (USA), Justin Rose (ENG), Tommy Fleetwood (ENG).
69 - Scott Hend (AUS), Paul Casey (ENG), Kiradech Aphibarnrat (THA), Ernie Els (RSA), Dustin Johnson (USA), Graeme McDowell (NIR), Gaganjeet Bhullar (IND).
70 - Jonas Blixt (SWE), Martin Kaymer (GER), Ryan Moore (USA), Ken Duke (USA), Huang Wen-yi (CHN), Jeong Jin (KOR), Kevin Streelman (USA), Peter Hanson (SWE), Louis Oosthuizen (RSA), Boo Weekley (USA), Sergio Garcia (ESP), Luke Donald (ENG), Darren Fichardt (RSA).
71 - Derek Ernst (USA), Peter Uihlein (USA), Lee Westwood, Graham Delaet (CAN), Ian Poulter (ENG), Billy Horschel (USA), Chris Wood (ENG), Hideki Matsuyama (JPN), Brian Gay (USA), Phil Mickelson (USA), Keegan Bradley (USA).