Six birdies on the back nine were not enough for Rory McIlroy to get close to Dustin Johnson, who won the WGC-Mexico Championship by five shots.

McIlroy did his best after making the turn one-over par, but though he made six birdies in seven holes before a bogey on the last, Johnson was every bit as good.

The American recorded five birdies in seven holes in a final-round 66 as he collected his 20th PGA Tour win and sixth World Golf Championship crown.

Things started well for McIlroy as he capitalised on an early bogey from Johnson to cut the gap further with a birdie on the second.

But dropped shots on the fourth and the sixth cost him momentum and by the time he rediscovered it, Johnson was matching him shot for shot.

The pair finished well clear of the rest of the pack, with McIlroy five shots ahead of a three-way share of third place, split between English duo Paul Casey and Ian Poulter, and Thai Kiradech Aphibarnrat.

"To get 20 wins out here is very difficult and to do it before I turn 35 is pretty incredible," the 34-year-old Johnson said in a greenside interview.

"It means a lot. This is a big win for me and it gives me a lot of confidence for the rest of the year."

Casey moved up 10 places on the leaderboard with a final round of 65, sparked by four birdies on the opening six holes.