Defending champion McIlroy shot a 66 to surge up the leaderboard at Bay Hill.
The former world number one, strong off the tee, birdied four of his first six holes and three of his final four as he continued solid play for the season.
McIlroy's round could have been even stronger had he not twice three-putted at par-five holes, including at the 12th where his putting cost him a chance of an eagle.
Australian Aaron Baddeley (69), Englishman Matt Wallace (69) and American Kevin Kisner (70) shared third at seven-under.
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Third round co-leaders Tommy Fleetwood (76) and Keegan Bradley (75) stumbled out of contention after beginning the day with a four-stroke advantage.
(Reporting by Gene Cherry in Raleigh, North Carolina, editing by Nick Mulvenney)
