(CNN) It could have been the perfect riposte to his Portrush pummeling but Rory McIlroy just couldn't hold off bulldozing Brooks Koepka in the World Golf Championships event in Memphis.

McIlroy led Koepka by a stroke going into the final round but instead of an epic duel with the world No.1 he faltered with a 71 as the American surged past with a 65 to win the WGC-FedEx St Jude Invitational by three from countryman American Webb Simpson.

The 30-year-old McIlroy, who had gone clear after a blistering 62 Saturday, ended in a tie for fourth, five strokes adrift.

McIlroy missed the cut at the Open in his native Northern Ireland last week after a disastrous opening 79, followed by a scintillating second-round 65, willed on by the vocal home fans as he tried in vain to make the weekend.

Brooks Koepka surged past Rory McIlroy with a final-round 65.

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