(CNN) -- Rory McIlroy's first round as new world number one ended in a disappointing one-over par 73 as Adam Scott and Jason Dufner set the pace in the WGC-Cadillac Championships at Doral in the U.S. state of Florida.
Paired with Luke Donald, who McIlroy deposed by winning the Honda Classic last Sunday, and third-ranked Lee Westwood, the Northern Irish star admitted to being "mentally flat."
"It was a pretty tricky day, but obviously there was good scores out there. I just didn't really get anything going," the 22-year-old said.
"All of a sudden you're there (top of the rankings) and you're like, 'Well, what do you do?' I just need to go out and set myself a target tomorrow."
Donald, who can regain top spot with victory come Sunday, played solidly for a two-under par 70, four behind the two leaders, but Westwood struggled to a 76.
Tiger Woods started with an eagle as he looked to build on his heroic final round of 62 at the Honda, but then struggled in the windy conditions on the famous Blue Monster course.
He made only one putt of over four feet and ended with a level par 72, the same mark as his old rival Phil Mickelson.
"It was just a difficult day," Woods said. "The wind was blowing putts around, and it made for a very challenging round.
"It was gusty and changed directions every now and then. For some reason I kept hitting every drive in the first cut, so it added and compounded the problem. I had to consequently play very conservative on a lot of the shots."
Meanwhile, Australia's Scott, who has played sparingly on the PGA Tour this season, powered to a six-under 66.
American Dufner matched him in the clubhouse with a superb birdie on his last hole, the ninth.
Thomas Bjorn of Denmark and in-form Masters champion Charl Schwartzel of South Africa were their nearest challengers after the par shot 68s.
England's Justin Rose, who was always a factor in last week's tournament, continued his fine form to be in an eight-strong group at three under which also included PGA champion Keegan Bradley.