Dustin Johnson won his first event since becoming World Number One as the American held off a strong European challenge at the WGC–Mexico Championship last March.The 2016 US Open champion recorded a closing 68 for a 14 under total at Chapultepec Golf Club.
Johnson became the fifth player to win their first event since becoming World Number One, following in the footsteps of Ian Woosnam, David Duval, Vijay Singh and Adam Scott as he captured a fourth World Golf Championships title – a tally bettered only by Tiger Woods with 18.
Johnson moved to the top of the Official World Golf Ranking only two weeks previously and conceded that had been on his mind.
"It means a lot because it's a tough spot to be in" Johnson said. "There's a lot of pressure on you and I came out and I played really well.
"I hit the ball great all week. The greens are tough to putt on and I didn't feel like I putted my best, but I really hit the ball well and played just well enough I guess because I won by one."
Fleetwood's performance took the 26 year old to a career-high of 35th in the world, with Rahm up to 25th.
Rory McIlroy began the final round two shots off the lead but was never able to mount a charge, the World Number Three finishing joint seventh after carding two bogeys and two birdies in a closing 71.