Brooks Koepka fired a six-under 66 to grab a one-stroke lead at the halfway mark of the WGC-Workday Championship.

The four-time major champion made seven birdies and one bogey at The Concession Golf Club in Florida on Friday to move into 11 under.

Koepka holds a 36-hole lead or co-lead on the PGA Tour for the eighth time in his career as he eyes a second World Golf Championships title.

The American made three straight birdies from the 15th before dropping his only shot of the round at the last.

Koepka is a stroke clear of Collin Morikawa (64), Billy Horschel (67) and Australian Cameron Smith (66).

Morikawa, last year's US PGA Championship winner, produced the equal best round of the day, with Bryson DeChambeau – who is tied for 20th – also shooting a 64.

Overnight leaders Webb Simpson and Matt Fitzpatrick both slipped back into a tie for fifth after firing 69s, sitting at nine under alongside Tony Finau (67).

Defending champion Patrick Reed carded another 68 to be at eight under alongside Kevin Kisner (69).

Rory McIlroy shot a two-under 70 to get to five under and into a tie for 13th, with Justin Thomas (66) also among that group.

A six-time WGC winner, Dustin Johnson improved on his opening-round 77, carding a 69 that sees him sitting at two over.