The World Number One won the WGC-Mexico Championship earlier that month and was in stunning form all week to emulate Tiger Woods and make it consecutive WGC wins.It was also his second consecutive European Tour win and took his tally of triumphs in both categories to five.
The American did not trail at any point in any match all week as he made it seven wins out of seven and lost just 23 of the 112 holes he played at Austin Country Club.
Rahm had also been in brilliant form in his first six matches. While he mounted a spirited fightback from five down after eight holes, he came up just short in an exhilarating encounter to make it two top threes since joining the European Tour on March 1.A 30-footer on the 16th had cut the gap to one, but two closing pars were good enough to hand Johnson the win.
"I'm proud of how I played, the way I stayed in there. He made birdies on the back nine but I just couldn't get any putts to drop."