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

Johnson won for the 20th time on the PGA TOUR, making him a lifetime member at age 34.

He felt just as good about the way his game is headed. Johnson had said on Thursday he was starting to swing it as well as he did two years ago, when he won three straight tournaments to become the dominant figure in golf until his spill down the stairs on the eve of the Masters led to a back injury.

“This gives me a lot of confidence for the rest of the year,” he said.

McIlroy was five behind at the turn and ran off six birdies in a seven-hole stretch before closing with a meaningless bogey for a 67. Johnson was right there with him with five birdies on the back nine.

The only drama came early.

Johnson had to make an 18-foot par on No. 2 to avoid a two-shot swing. He had to make a 6-foot putt to salvage bogey on the next hole, and his lead was cut in half in just three holes. Leading by three, the tournament turned on two holes and two trees.

Johnson hit iron off the tee to the right at No. 5 and his ball settled right behind a tree. He called for a ruling, and showed the official that with a shot slightly behind toward the fairway, his right heel was on the curb of a cart path.

He was given free relief, carved a punch shot under the tree limbs and two-putted from 50 feet for an unlikely par.

“I got a fortunate break there on No. 5 and ended up making a nice par,” he said. “Sometimes the rules work to your advantage.”

On the par-5 sixth, McIlroy went left and his ball settled behind a tree. He asked for relief and was denied. McIlroy said he wanted to chip away from the fairway to an open area, and when he showed European Tour official Mike Stewart his intended shot, his right foot was on the path. However, to play at that angle, a 4-inch nob on the trunk blocked the path of his club, meaning McIlroy wouldn’t be able to make contact.

To play out left-handed, McIlroy would have had to stretch his foot to reach the path. In both cases, relief was denied. Stewart said he asked McIlroy if he wanted a second opinion, and McIlroy declined. He punched out left-handed, and his third shot went into the water, leading to a bogey.

Johnson made an 8-foot birdie, the lead was up to five shots and Johnson took it from there.

Johnson finished at 21-under 263, the lowest score by five shots in the three years at Chapultepec Golf Club. McIlroy continue his solid run. This was his fourth consecutive finish in the top 5, the longest such streak of his career.

Paul Casey (65), Ian Poulter (68) and Kiradech Aphibarnrat (68) tied for third, though they were 10 shots behind.

Tiger Woods got his first top 10 of the year, but it wasn’t all that inspiring. He again struggled on the poa greens of Chapultepec. Over the last two rounds, Woods hit 31 out of 36 greens in regulation and only had scores of 70 and 69 to show for it.

The best round belonged to Justin Thomas, who closed with a 62 before heading off to the Honda Classic to defend his title. Thomas was 8 under through 10 holes when his tee shot on the par-3 third hole landed inches behind the hole and spun 40 feet away.

Even so, this day was all about Johnson.

It was the eighth time he has built a 54-hole lead of three shots or more, including one year at Pebble Beach when the final round was rained out. Johnson only gave McIlroy a glimmer of hope early, and no hope on the back nine as they matched birdies on three straight holes through the 16th.