The Northern Irishman entered the day at TPC Southwind five shots off the lead but carded his lowest ever round in a World Golf Championships event and his lowest of the season on the European Tour to get to 12 under par and lead the way from World Number One Brooks Koepka.
The two four-time Major champions will now go out in the final group on Sunday in what promises to be a scintillating final day as the tournament makes its first appearance in Tennessee.
McIlroy won this event in 2014 when it was played at Firestone Country Club and will be looking for a third World Golf Championships win, while Koepka will be seeking his first, as will overnight leader Matt Fitzpatrick of England. The Englishman carded a round of 69 to sit in third place at ten under par, a shot clear of Swede Alex Noren, Spaniard Jon Rahm and Australia’s Marc Leishman.
American Billy Horschel, Dane Thorbjørn Olesen, and Ian Poulter of England are then at eight under par, while his countryman Tommy Fleetwood is in a tie for tenth place alongside American trio Webb Simpson, Justin Thomas and Bubba Watson at seven under par.