McIlroy lost the No. 1 position in March 2013 when his game was in a downward spiral as he was adjusting to a new equipment deal and going through another management change. But since winning the BMW PGA Championship at Wentworth at May, his game looks as strong as ever.
"Obviously, Rory is in incredible form at the moment," Scott said.
Garcia was a runner-up to McIlroy for the second straight time.
The Spaniard had the daunting task of making up a seven-shot deficit at the Open Championship, and Garcia put up a great fight until finishing two shots behind at Hoylake. Staked to at three-shot lead at Firestone, it didn't go much better.
Garcia missed a 6-foot birdie on the par-5 second hole with a putt that never looked as if it was going in. His lead down to one, Garcia pulled his tee shot into the gallery on the third hole, striking a woman on the hand and knocking the diamond out of her ring. The diamond was found, about the lone bright spot in his day. Garcia made bogey, and McIlroy rolled in another birdie putt to take the lead.
Garcia made only one birdie, on the ninth hole to share the lead. McIlroy answered with an 8-foot birdie on No. 11 and was on his way.
Marc Leishman (67) finished alone in third.
Patrick Reed holed out for eagle on No. 17 in his round of 65 and gave him a tie for fourth, enough to move him up to No. 7 in the Ryder Cup standings and boost his hopes of making his first team with only the PGA Championship left in the qualifying period.
Phil Mickelson made 10 birdies for a career-best 62 at Firestone.
None of that could top McIlroy, who put on another exhibition and looked every bit the part of the world's best player.
"He'll be the man to beat next week by the look of things," Scott said. "And I'll be gunning for him, for sure."