Morikawa made six birdies on the front nine, including a run of six in seven holes from the third to ninth, and ended the round at 15 under, despite a level-par back nine.

Koepka, who started round three one shot ahead of Morikawa with an outright lead, bogeyed the first two holes, but fought back to shoot a two-under 70 and is tied for second place with Billy Horschel at 13 under.

Webb Simpson is a further shot back with the top four players al American.

Northern Ireland's Rory McIlroy, who is chasing a WGC grand slam having previously won the other three tournaments in this series, shot 66 - the lowest round of the day - to move to 11 strokes under par and be tied for fifth.

McIlroy did that in spite of a double bogey on the second hole.

American Patrick Reed, the defending champion, is also tied for fifth place.

It is a high-quality group at the top of the leaderboard, with Horschel the only player in the top six who has not won a major.

England's Matthew Fitzpatrick, Japanese player Hideki Matsuyama, Scottie Scheffler of the United States and Norwegian Viktor Hovland all find themselves five shots off the lead with 18 holes left to play.

The final round is to be played tomorrow.

This tournament - moved from Mexico to Florida because of the coronavirus pandemic - is the first WGC event of the season.