Dustin Johnson closed to within four shots of Brooks Koepka in the final round of the US PGA Championship to raise hopes of an unexpectedly dramatic finale at Bethpage Black.

Koepka began the day leading by seven strokes at 12 under and seemingly set to cruise to a fourth major title inside two years, with Johnson one of four men sharing second.

However, while the reigning PGA champion failed to improve on his score over the front nine, fellow American Johnson picked up three shots to reach eight under.

No other player was remotely close to Koepka, who missed a number of fairways early in his round but generally found himself far enough down holes to leave approaches relatively simple.

A sloppy bogey at the first for the runaway leader was cancelled out by a routine birdie at the long fourth, where he reached the green in two before two-putting.

Koepka then rounded out the outward nine with five pars, safe in the knowledge that he did not need anything spectacular. While his play from tee to green was not as stunning as earlier in the week, his distance control on the greens was exceptional and he remained firmly in control of his own destiny.

Amid gusting winds in New York, Johnson - the 2016 U.S. Open winner - was playing superbly. He birdied the fourth, sixth and ninth, with a couple of other opportunities going begging on the greens.

Matt Wallace and Jazz Janewattananond shared third, seven strokes behind Koepka at five under par.