Overnight leader Dustin Johnson survived an early wobble to win the BMW Championship at Crooked Stick and move top of the FedEx Cup standings with one event to go.

The US Open champion won by three shots, having initially extended his advantage to four at the opening hole only to bogey the next two to find himself in a tie with in-form Englishman Paul Casey.

However, the world number two then reeled off five birdies in his next nine holes to pull clear of his challengers and, although a bogey at the par-three 13th cut his advantage to three, he won comfortably after a five-under-par final round of 67 to close 23-under.

Victory puts him in pole position heading into The Tour Championship in Atlanta later this month, the week before the Ryder Cup, in which Johnson will be one of United States captain Davis Love III's key players.

Casey finished second also after a round of 67 which included four birdies and an eagle at the par-five 15 - but that was matched by Johnson to cancel out the benefit.

American Roberto Castro finished third a further three stokes back.

Rory McIlroy signed off with a 72 to end his tournament on three-under, 20 shots off the winner in a tie for 42nd.

His compatriot Graeme McDowell sat beside after also closing with a 72.

Defending champion and world number one Jason Day withdrew mid-round complaining of a back problem, while the likes of Rickie Fowler - now relying on a captain's pick on Monday to make the USA team for Hazeltine - Sergio Garcia and Henrik Stenson all dropped out of the top 30 and will miss the final event of the PGA Tour at East Lake.