SYDNEY, 27 Nov 2014 (AFP) - The 21-year-old American holed six birdies and two bogeys in a four-under 67 to lead Australians Aron Price and Scott Gardiner.
Spieth stole the opening round headlines from world number one McIlroy and third-ranked Scott in tricky conditions at the Jack Nicklaus-designed The Australian course.
Jet-lagged McIlroy believes his best is yet to come as he defends the title he won last year after opening with four birdies and two bogeys in an opening 69 to be two strokes adrift of Spieth.
But it was a disastrous start by Scott, who finished three over 74 and in some danger of missing Friday's halfway cut after a double-bogey, three bogeys and just two birdies.
Spieth comes into the event as a runner-up at the Masters last April behind team-mate Bubba Watson, just missing out on being the first Masters rookie to win since Fuzzy Zoeller in 1979.
"Overall, it was a solid round, I putted great, my short game kept me in today," Spieth said.
"I don't know how many times I've led after round one, but my approach tomorrow is just trying to keep it consistent, don't worry about the scoreboard and let my putter do the work."
Northern Irishman McIlroy, who flew in late Monday from Dubai where he was runner-up to Sweden's Henrik Stenson in last weekend's DP World Tour Championship, was looking for some rest after a 4:30am wake-up call for his early start at The Australian.
"I definitely feel like there is a better score out there," he said. "I gave myself a lot of chances early on in the round but didn't really take them.
"I started off OK but in the middle of the round I hit a couple of loose shots and actually made a couple of good up-and-downs for pars, but then made three birdies on the back nine against one bogey.
"Even though I felt I didn't play that good today anything in the 60s was a pretty good score out there."
McIlroy is coming off a stellar year, winning two majors (the British Open and the US PGA Championship), a World Golf Championship title (WGC-Bridgestone) and the flagship event of the European Tour (BMW PGA Championship) to finish the season at the summit of the world rankings.
McIlroy overhauled Scott with a birdie at the final hole to win last year's Australian Open at Royal Sydney in what was his only 2013 tournament success.
Scott was in strife from the start in Thursday's round with a double-bogey at the first hole on the way to dropping five shots on his opening nine holes.
He clawed back two shots coming home and his putt for birdie lipped out at the last hole to be seven behind Spieth.
It was a big day for Gardiner, who landed himself a car with a spectacular hole-in-one, trickling down the slope on the 176-metre par-three 11th hole.
Japan's Achi Sato joined Spieth in the lead late in the round only to drop back to two-under with two bogeys.
Zimbabwe's Brendon de Jonge was in a group on two-under with McIlroy after six birdies and four bogeys and temporarily threatened the lead during his round.
Leading scores after the opening round of the Australian Open at The Australian course (par-71) in Sydney on Thursday (Australia unless stated):
67 - Jordan Spieth (USA)
68 - Aron Price, Scott Gardiner
69 - Adam Crawford, Rory McIlroy (NIR), Richard Green, Brendon De Jonge (ZIM), Andrew Tampion, Geoff Drakeford, Achi Sato (JPN).
70 - Brett Rumford, Bryden Macpherson, Antonio Murdaca, Patrick Rodgers (USA), Rohan Blizard, Conrad Shindler (USA), Steven Bowditch, David McKenzie, Michael Hendry (NZL), Rhein Gibson, Lincoln Tighe
71 - Jin Jeong (KOR), Greg Chalmers, Josh Younger, Todd Sinnott, James Nitties, Robert Allenby, Jake Higginbottom, Ryan Ruffels, Oliver Goss, Tae Koh (NZL)
72 - Rory Hie (INA), Nathan Green, Joon-woo Choi (KOR), Mathew Goggin, Mahal Pearce (NZL), Jason Scrivener, Bradley Hughes, Ryan Fox (NZL), Matthew Griffin, David Bransdon, Boo Weekley (USA), Stephen Dartnall, Chris Campbell, Matt Jager
Selected scores:
73 - Sung-Hoon Kang (KOR), Garrett Sapp (USA), Gunn Yang (KOR), John Young Kim (USA), Devin Daniels (USA), Ryan C Carter (USA), Daniel Woltman (USA)
74 - Jin-ho Choi (KOR), Ju-hyuk Park (KOR), Yi-keun Chang (KOR), Adam Scott, Kyle Stanley (USA), Wisut Artjanawat (THA), Kyong-hwan Lee (KOR), Minghao Wang (CHN)
75 - Chirat Jirasuwan (THA)
76 - Zac Blair (USA), Bi-O Kim (KOR), Tommy Mou (TPE)
77 - Xinyang Li (CHN), Do-hoon Kim (KOR), Jamie Lovemark (USA), Deuk-hee Park (KOR), Hyo-won Park (KOR)
78 - Wen-yi Huang (CHN), Jin Zhang (CHN)
79 - Sam Yi (USA)
80 - Jordan Krantz (CAN)
83 - Michael Im (KOR)
84 - Weiyu Zhu (CHN)