MEXICO CITY (AP) — Dustin Johnson arrived in Mexico trying to figure out what was wrong with his swing. He left Sunday night with his 20th career PGA Tour victory, his sixth World Golf Championship and now wonders how far that will take him.

Over 72 holes, only two were worse than par. No one got closer than two shots to him all weekend.

Johnson overcame a sloppy start, caught a good break from behind a tree and made the rest look easy, just like when he was on top of his game two years ago. He closed with a 5-under 66 for a five-shot victory over Rory McIlroy in the Mexico Championship.

"This is a big one for me, and it gives me a lot of confidence for the rest of the year," Johnson said. "I feel like the game is in good form right now."

Johnson becomes the 38th player in PGA Tour history with 20 victories, which makes him a lifetime member when he puts in 15 years.

He won on both sides of a four-week journey, which started with his victory in the Saudi International, and he will return to No. 1 in the world next week.

McIlroy did all he could, making six birdies in a seven-hole stretch on the back nine, and closed with a 67.

"I was making birdies and going nowhere," McIlroy said. "He just played great this week. Hats off to him. He's arguably the best player in the world. And he's got two wins already this year, and he has been one of the best players for a long time."

Johnson won the Mexico Championship for the second time in three years. It was in 2017 when he was playing the best golf of his life, with three straight victories against the strongest fields, only to suffer a back injury when he slipped down the stairs in his rental house on the eve of the Masters.

"It's the closest I've felt to that since I got hurt two years ago," Johnson said. "I finally feel like everything is getting very close to where I was then. ... If I know I'm hitting it well and driving it good, it's going to give me a lot of confidence."

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 to keep his lead at two. McIlroy made bogey on the next hole, and then 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 6-iron 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 he was on his way.

"I could definitely see where Mike was coming from," McIlroy said. "I was taking a stance that was hitting it one way, and my foot was on it. But I could sort of see his point, as well. That's why I didn't call for a second opinion. It wasn't going to make much of a difference anyway. I was looking left, and I didn't have much of a shot from there, either."

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

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.

For the second straight day, he declined to speak to reporters and gave comments only to a PGA Tour official.

The best round belonged to Justin Thomas, who closed with a 62 before heading off to the Honda Classic to defend his title.

Even so, this day was all about Johnson. He is off for two weeks before returning at The Players Championship, the Dell Match Play and the Masters. He's not about to relax now.

"There's too many important tournaments coming up," Johnson said. "I want to keep my game as sharp as I can."