Extra holes looked on the cards when Rahm drove to the back edge of the 18th green, but a bad chip took the pressure off Johnson and he safely made par from the fringe to secure the title.
Johnson won the WGC-HSBC Champions in 2013, the Cadillac Championship in 2015, and has now won three of the last four - the 2016 WGC-Bridgestone Invitational, the 2017 WGC-Mexico Championship (formerly known as Cadillac), and the 2017 WGC-Dell Technologies Match Play Championship. He has two top-five finishes in his past three tries at the Shell Houston Open, so course history combined with recent form will make him one of the favorites to come out on top.
A birdie on the 15th was followed by a remarkable one on the 16th, Rahm hitting his second shot seemingly straight through a tree before holing from 30 feet to get back to just one down.
"I think Jon is already one of the 10 best in the world", Mickelson said. He ran it so far past the hole that the ball actually ran off the green. As a result, Johnson will meet Hideto Tanihara of Japan in today's semi-final. And in a tournament record that will be tough to top, Johnson led after 94 per cent of the holes he played - 105 out of 112. "But at that point I was ready to sign my scorecard and get my trophy, so I just said, give me the penalty, let's go". "I feel like everything is working pretty well", said Johnson in something of an understatement.
It's the first time he's played the 17th hole in any of his 6 matches.
Johnson was 4 up with six holes to play when the Spanish rookie with a big game and no fear hit driver over the water and onto the 13th green to win the hole with a birdie. "There's still a lot of room for improvement in my game, so I'm just going to keep working on it". A missed six-footer for birdie on the fifth hole cost Rahm again, and then, on the par-5 sixth hole, Rahm missed the green badly with his second shot and couldn't get up and down to halve the hole after Johnson made birdied, giving DJ a 4-up lead. The American beat Spaniard Jon Rahm in the final to claim his third tournament victory in a row and climb to the summit of the FedEx Cup standings. "I definitely have what it takes to win out here". Tanihara did have the highlight of the day though, acing a hole-in-one on the 7th against Haas in the consolation match.