Halfway leader Shane Lowry fell six shots off the pace after the third round of the Arnold Palmer Invitational.
The Offaly man began the day with a two-shot advantage, but struggled to an error-strewn 76 at Bay Hill which left him at four under par.
The former Open champion bogeyed the first two holes and although he bounced back with birdies on the sixth and seventh, double bogeys on the 11th and 13th left him with a mountain to climb on Sunday.
Two-time major champion Collin Morikawa carded a superb 67 to take over at the top of the leaderboard on 10 under par, a shot ahead of fellow American Russell Henley.
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Henley holed out from a bunker for a birdie on the first and had four more in succession from the 10th as he matched Morikawa's 67.
Morikawa missed the cut on his last two starts at Bay Hill, but believes he has "more control" over his game as he bids to claim a seventh PGA Tour title.
"You have to hit good shots out here," Morikawa said. "It can kind of get you from any direction, but I felt like I've had control the first three days and just got to keep sticking to that game plan for tomorrow."
Told by CBS interviewer Cara Banks that it has been more than 500 days since his last victory, Morikawa joked: "Thank you for putting that in exact days - I hope you have the hours and minutes!
"I've said it since day one, stick to my game plan.
"I know how to win out here, I've done it before but honestly tomorrow, for 18 holes I just have to put it out there every single shot, not get ahead of myself, not think about the past and just be in control of who I am."
Canada's Corey Conners lies two shots off the lead following a 69, with former world number one Jason Day another stroke back after completing his own 69 with three straight birdies.
Rory McIlroy bogeyed three of his last four holes to return a disappointing 73 which left him on three under alongside Ryder Cup team-mate Robert MacIntyre and England's Aaron Rai, with world number one Scottie Scheffler and Justin Rose on two under following matching rounds of 71.
