Dustin Johnson

Johnson appeared set to cruise to victory when he stormed into a five-up lead after only eight holes in the final, but Rahm staged a dramatic fightback to take the power-hitting contest to the final hole of a long week at the Austin Country Club.

But the world No 1 held his nerve to match Rahm's par at the last to seal a one-up win and become the first player to win all four current World Golf Championship titles.

Johnson was four shots clear of the field early on the back nine at Club de Golf Chapultepec in Naucalpan before back-to-back bogeys opened the door for the likes of Jon Rahm, Tommy Fleetwood and Ross Fisher, but the American held his nerve down the stretch to collect his fourth World Golf Championship title.

Fleetwood continued his impressive start to the year as he won the race for second by holing a huge putt for birdie on the final green, while Rahm was left to rue a poor finish as he three-putted the 16th and 17th greens to slip to 12 under alongside Fisher.

Dustin Johnson moved into the outright lead at the Tour Championship after carding a three-under 67 in the second round.

The American U.S. Open champion sits at seven under at East Lake, a shot clear of countryman Kevin Chappell (68).

The duo have broken away from the rest of the field, with Kevin Kisner (70) and Hideki Matsuyama (71) at three under.

There is also one less contender after world number one Jason Day withdrew, citing back pain.

Dustin Johnson grabbed a three-shot lead at the BMW Championship after birdieing four holes late in his third round on Saturday.

Crooked Stick Golf Club, a Pete Dye design, played tougher in round three with winds up and fairways providing minimal roll.

Johnson struggled to get anything going early, but he caught fire on the back nine, setting himself up for a chance to earn his third win of the season.

The American sits at 18 under, three shots clear of Englishman Paul Casey, after both carded four-under 68s.

Johnson played his last seven holes knowing he had to review a possible rules infringement after the round.

He shot 68 to finish five under but was then penalised a shot for making his ball move on the fifth green, despite being initially absolved of wrongdoing.

McIlroy called organisers "amateur" and Spieth said the decision was "a joke".

Johnson's score was amended to four under, but he still finished three shots ahead of Irishman Shane Lowry and American duo Jim Furyk and Scott Piercy at Oakmont.