Rory McIlroy remarkably overturned a six-shot deficit in the final round of the Tour Championship on Sunday to become the first player to win the FedExCup three times.

McIlroy carded a closing 66 at East Lake to finish 21-under-par, a shot ahead of Sungjae Im and Scottie Scheffler, who had been an odds-on favourite to secure his fifth win of the season and the $US18 million ($A26m) first prize.

However, Scheffler struggled to a 73 and McIlroy took full advantage to claim an extraordinary victory, having started the week six shots behind the World No.1 under the handicap scoring system and making a triple bogey on his first hole.

"Two of the best players in the world going head on the best tour," McIlroy said of the head-to-head battle.

"I believe in the game of golf. I believe in this Tour, in particular. I believe in the players on this Tour. It's the greatest place in the world to play golf, bar none, and I've played all over." - Rory McIlroy.

McIlroy has now earned $US43 million ($A62m) for his three FedExCup victories. Tiger Woods (2007, 2009) is the only other player to have won it more than once.

Australia's Cameron Smith, The Open champion reportedly on the verge of joining the Saudi-backed LIV Golf Series, finished 20th at nine-under after his final-round one-under 69 while compatriot Adam Scott (70) was 25th at four-under.

Playing together in the final group, McIlroy and Scheffler both bogeyed the 1st and while Scheffler also dropped shots on the 4th and 6th, McIlroy birdied the 3rd and made three more in succession from the 5th for a share of the lead.

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Scheffler responded immediately with a birdie on the 8th and although McIlroy got back on level terms at the 12th, the Northern Irishman bogeyed the 14th after missing the green with his approach.

McIlroy was not about to throw in the towel though and holed from 30 feet for birdie on the 15th and saved par on the next after flying his second shot over the green from a fairway bunker.

The former World No.1's chip was travelling at speed when it clattered into the pin and, after Scheffler made bogey from a greenside bunker, McIlroy holed from seven feet to take the lead for the first time.

A poor bunker shot on the par-5 18th cost Scheffler the chance to force a play-off and the Masters champion had to scramble for par to share second place with South Korea's Im, who matched McIlroy's closing 66.

"I believe in the game of golf. I believe in this Tour, in particular. I believe in the players on this Tour," McIlroy said. "It's the greatest place in the world to play golf, bar none, and I've played all over."