Jason Day has imploded to hand Dustin Johnson the World Golf Championships event in Ohio.

The world number one has double bogeyed the 16th hole and bogeyed the 15th and 18th to fall to third, three shots behind Johnson.

It's Johnson's first start since winning the US Open last month.

He's finished a shot clear of fellow American Scott Piercy, with Jordan Spieth amongst those finishing in a tie for third with Day.

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New Zealand number one Danny Lee has finished 56th in the elite 61-player field, 27 shots behind Johnson.