“I know what it takes to win out here and I definitely have what it takes to win out here, but I still feel like I’m not playing my best golf,” Johnson said after defeating Rahm 1up.

Rahm, a powerful 22-year-old who won the Farmers Insurance Open in March, held a perfect record for the week as he entered the championship match, but was overcome by the World No.1 on the final hole.

Johnson was five up after eight holes and looked to be cruising before the Spaniard birdied three of the next four holes, before taking Johnson down the 18th for the first time all week.

“I’ll be sad in a couple of minutes, it’s understandable, but after that I’ll be really happy. It’s all good to come,” Rahm said of the result.

Jon Rahm pushed Dustin Johnson all the way in the final. PHOTO: Richard Heathcote/Getty Images.

Johnson, who also claimed the WGC Mexico Championship in March, won all seven of his matches and didn’t fall behind in any of the 112 holes he played this week – an impressive turnaround after he entered the week with an 11-win, 10-loss match play record.

“There’s a lot of great players and I’m the first one to do it,” Johnson said after completing the WGC Slam. “I feel like there will be some more to do it, but I’m very, very pleased with it.”

FINAL LEADERBOARD

Johnson’s other two WGC victories came at the Bridgestone Invitational in March, 2016, and the HSBC Champions in November, 2013. The new leader of the FedExCup now heads to Texas for the Shell Houston Open before heading to Augusta as the favourite to don the green jacket.

“The Masters is always a tournament I look forward to every year,” Johnson said. “I really like that golf course … I really like going to Augusta. How do I like (being favourite)? I don’t care.”

American Bill Haas finished third after overcoming Japan’s Hideto Tanihara 2 & 1 in the consolation match.

After Jason Day’s withdrawal early in the first round, Marc Leishman was the sole Australian left in the field. He survived a three-way play-off to advance from the round robin group stage but was knocked out in the Round of 16 by a red-hot Phil Mickelson. The veteran birdied four of the first nine holes to lay the platform for a 4 & 3 victory.