However, the 43-year-old then found water with his approach to the sixth and missed from three feet for bogey, before taking four to get down from off the green on the par-three seventh.

Consecutive double bogeys dropped Westwood all the way back to seven under par and six off the pace being set by world number one Johnson, whose place at the top of the rankings was in jeopardy if McIlroy could claim a third WGC title.

Johnson began the final round a shot behind fellow American Justin Thomas, who doubled his advantage with a birdie at the first before dropping a shot on the second.

Playing partner Johnson birdied the same hole to draw level and picked up another shot on the par-five sixth, before Thomas found water with a terrible tee shot on the seventh to run up a double bogey.

That left Johnson on 13 under par and two clear of Belgium's Thomas Pieters - the pair finished in the same positions in the Genesis Open a fortnight ago - with Spain's Jon Rahm alongside Thomas on 10 under.

McIlroy was a shot further back after two bogeys and a birdie in his first seven holes, the world number three three-putting the seventh from long range.

Johnson was attempting to become 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.

And the 32-year-old was looking in imperious form as he birdied the eighth and ninth to extend his lead to four shots with just nine holes to play.

McIlroy was now six shots off the pace and not helped by seeing his drive on the ninth come to rest in an old divot, the Northern Irishman slamming his club into the ground in frustration after that contributed to a mediocre approach.

Westwood's miserable day went from bad to worse with a third double bogey in the space of four holes on the ninth, where he four-putted from 45 feet.