Dustin Johnson will take a commanding six-shot lead into the final round of the WGC-HSBC Champions after Brooks Koepka's challenge faded in Shanghai on Saturday.

Johnson started moving day with a one-shot advantage over fellow-American Koepka after carding a sublime 63 on Friday and he followed that up with a four-under 68 to go 17 under.

The world number one recovered from a bogey at the first to reel off four birdies on the front nine before losing his way with a double-bogey at the 10th.

Another three gains on the back nine enabled Johnson to have a fifth title of the year firmly in his sights as compatriot Koepka failed to build on a promising start to his third round.

The U.S. Open champion led by two shots after stating his intent with birdies at the first three holes, but a triple-bogey took the wind out of his sails.

Koepka fired a wild tee shot left into the trees and was drawn to the foliage again with a poor approach at a horror eighth hole, going on to drop another three shots on the back nine and make only two birdies to sign for a one-over 73.

Henrik Stenson moved up to third with a three-under 69, dropping just the one shot at eight to deny himself a share of second spot with Koepka.

Brian Harman and Justin Rose are a further stroke back following third rounds of 70 and 72 respectively.

Patrick Reed had a day he will want to forget in a hurry, plummeting down the leaderboard following a 10-over 82 which included a triple-bogey and four double-bogeys.