Former world number one Spieth could only manage a 69 as Luck came out on top of an illustrous threeball.
Luck shares the lead with fellow 20-year-old Lucas Herbert, after the two Australian young guns posted superb five-under par opening rounds of 67.
Former world number one Spieth could only manage a 69 as Luck came out on top of an illustrous threeball that also included 2006 US Open champion Geoff Ogilvy, who carded a 70.
World number seven and Australian favourite Adam Scott struggled to a one-over 73 later in the day.
Luck carded seven birdies and impressed world number five Spieth. "I thought he was better composed than I was, no doubt," the American said.
"Around the greens, he was spectacular, which is something that you certainly need to take with you up to the next level."
Starting at the 10th, Luck was far from overawed by playing alongside major winners Spieth and Ogilvy, completing his first 12 holes in four under par.
He lost momentum with consecutive bogeys but rebounded immediately with three straight birdies and a closing par.
"I don't think I did anything outrageously good to get where I was," said Luck, who is already assured of places in three majors next year -- the US Masters, US Open and British Open.
Spieth, the 2014 Australian Open champion, had only one blemish on his card when a misjudged flop shot at the eighth saw the ball roll back to his feet and resulted in a bogey five.
"I made a couple of putts from outside five or six feet but that was it, " said Spieth. "But they'll start falling. If I can shoot three under each day... then I'll be in good shape."
Scott drained two long birdie putts at the par-three sixth and 17th, but five bogeys spoiled the 2013 US Masters champion's day.
"I didn't drive it well, hit the irons well, chip it well or putt well today, it was all very average," Scott said.
Herbert was bogey-free but missed a putt from three feet on the 18th that would have given him the outright lead.
The top three players in Sydney who are not otherwise exempt will win direct entry into next July's British Open at Royal Birkdale.