MIAMI, March 8, 2012 (AFP) - Scott, making just his third start of the season, had four birdies in his first nine holes and finished with five birdies and an eagle against one bogey in his six-under par 66.
America's Dufner nabbed four of his seven birdies on his last nine holes, including a birdie at his last, the par-three ninth, where he hit his tee shot within three feet of the pin to join Scott atop the leaderboard.
Northern Ireland's Rory McIlroy, making his first start since ascending to number one in the world rankings, opened with a one-over par 73.
Tiger Woods, who finished runner-up to McIlroy in the Honda Classic on Sunday, tapped in for eagle at his opening hole but found the going tougher the rest of the way en route to an even par 72.
Phil Mickelson, back in action after a couple of weeks off, also posted a 72.
Scott and Dufner had a two-shot lead over Denmark's Thomas Bjorn and South African Charl Schwartzel, who were tied on 68.
Spain's Alvaro Quiros and Miguel Angel Jimenez, England's Justin Rose, Juvic Pagunsan of the Philippines, Australian Aaron Baddeley and Americans Kyle Stanley, Keegan Bradley and Steve Stricker were on 69.
England's Luke Donald, supplanted by McIlroy atop the world rankings this week, headed another group of eight players on 70.
Leading first-round scores here on Thursday in the $8.5 million World Golf Championships Cadillac Championship at Doral (USA unless noted, par-72):
66 - Adam Scott (AUS), Jason Dufner
68 - Thomas Bjorn (DEN), Charl Schwartzel (RSA)
69 - Alvaro Quiros (ESP), Kyle Stanley, Keegan Bradley, Aaron Baddeley (AUS), Justin Rose (ENG), Juvic Pagunsan (PHI), Steve Stricker, Miguel Angel Jimenez (ESP)
70 - Charles Howell, Johnson Wagner, Peter Hanson (SWE), Luke Donald (ENG), Bubba Watson, Anders Hansen (DEN), Paul Lawrie (SCO), Zach Johnson
71 - Greg Chalmers (AUS), Gary Woodland, Hunter Mahan, Nick Watney
72 - Phil Mickelson, Fredrik Jacobson (SWE), Matt Kuchar, Jonathan Byrd, Yang Yong-eun (KOR), Mark Wilson, Jbe' Kruger (RSA), Martin Laird (SCO), Tiger Woods, David Toms