Dustin Johnson struggled early, caught a good break from behind a tree and then breezed to his sixth World Golf Championship title on Sunday by closing with a 5-under 66 for a five-shot victory over Rory McIlroy in the Mexico Championship.

He started the final round with a four-shot lead. But the lead cut in half after three holes, and Johnson escaped potential trouble on the fifth when he was given free relief behind a tree because the heel of his shoe was on the cart path.

McIlroy asked for similar relief on the next hole and was denied when it was deemed he could not attempt the shot. He wound up hitting into the water and made bogey. Johnson birdied and was on his way.

U.S golfer Dustin Johnson looks at his notes at the first hole during the WGC-Mexico Championship at the Chapultepec Golf Club in Mexico City, Sunday, Feb. 24, 2019. (AP PhotoChristian Palma)

U.S golfer Dustin Johnson looks at his notes at the first hole during the WGC-Mexico Championship at the Chapultepec Golf Club in Mexico City, Sunday, Feb. 24, 2019. (AP PhotoChristian Palma)

McIlroy closed with a 67.

U.S. golfer Dustin Johnson watches his ball in the second hole during the WGC-Mexico Championship at the Chapultepec Golf Club in Mexico City, Sunday, Feb. 24, 2019. (AP PhotoChristian Palma)

U.S. golfer Dustin Johnson watches his ball in the second hole during the WGC-Mexico Championship at the Chapultepec Golf Club in Mexico City, Sunday, Feb. 24, 2019. (AP PhotoChristian Palma)
U.S. golfer Tiger Woods bends down to observe from the first hole during the WGC-Mexico Championship at the Chapultepec Golf Club in Mexico City, Sunday, Feb. 24, 2019. (AP PhotoChristian Palma)

U.S. golfer Tiger Woods bends down to observe from the first hole during the WGC-Mexico Championship at the Chapultepec Golf Club in Mexico City, Sunday, Feb. 24, 2019. (AP PhotoChristian Palma)