Two weeks after McIlroy staved off a final round challenge from Garcia to win the British Open, the Northern Irishman will be the hunter instead of the hunted.
Garcia, coming off a course record-equalling 61, was solid rather than spectacular as he fashioned a three-under-par 67 at Firestone Country Club in Akron, Ohio.
He posted a 14-under 196 total with one round left, while McIlroy birdied the final two holes, rolling in a 35-footer at the last to card a 66 for 11-under.
Not that it was only a two-horse race after a round that was interrupted for more than three hours by storms.
Marc Leishman was four shots behind, while another Australian, world number one Adam Scott, was five back with Englishman Justin Rose and American Keegan Bradley.
"I misread a couple of putts here and there and hit a couple of not so good putts," Garcia told CBS television, otherwise expressing overall satisfaction with his game and welcoming the chance to potentially duel again with McIlroy.
"I've felt very comfortable throughout the whole week so hopefully I'll be able to do the same thing (on Sunday), just like he (McIlroy) did to me at Hoylake and hopefully keep him just behind me."
McIlroy for his part was relishing the chance to pursue the man with whom he will form the backbone of the European team for next month's Ryder Cup.
"Sergio was chasing me down at Royal Liverpool. I'm going to try and chase him down this week," said McIlroy, who can displace Scott at the top of the world rankings on Sunday.
McIlroy must win and Scott finish worse than fifth for this to happen.
The presence of four of the world's five top-ranked players on the leaderboard overshadowed the continuing struggles of Americans Tiger Woods and Phil Mickelson, the most successful two players of the past two decades.
They were 15 and 17 strokes behind respectively, and have only one more competitive round to find their game before next week's PGA Championship.
"I did not putt well again," said Woods, who has won eight times at Firestone.
On the positive side, he said he was starting to feel more comfortable with his tee shots after switching drivers this week.
"I went back to my old driver from last year. It has a lighter shaft in it. I figured the lighter shaft should help me out a bit, get my speed back, because I'm not quite as explosive as I need to be."
He was referring to his recovery from March back surgery, which kept him out of competition for four months.
Woods also commented on Dustin Johnson, the eight-time tour winner who has announced he is taking a break from competition to address "personal challenges."
"It's sad he's got to take a hiatus from the game of golf," Woods said. "He's such an enormous talent. It's always tough to see when one of your friends is struggling like that."
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