Florida native Daniel Berger moved five strokes clear ahead of the final day at the Honda Classic, matching the largest 54-hole lead in tournament history on Saturday.
The world number 20 had held a three-shot lead at the halfway mark but extended that with a one-under-69 following back-to-back 65s at Palm Beach Gardens in Florida.
Berger led by as many as six strokes after sticking a brilliant tee shot for birdie on the par-three 15th hole before a bogey on the 18th hole.
The American leads from a group of four players tied on six-under, including Irishman Shane Lowry who carded a round of six-under-67 to move up the leaderboard.
Lowry is tied with Sepp Straka, Chris Kirk and day one leader Kurt Kitayama, with the latter two carding rounds of one-over-71 having been tied for second after the first two days.
“Obviously you want to go out and catch him tomorrow, but I don’t think you can go and catch anyone on this golf course," said Lowry, whose round was the best of the day with only 13 players above even.
“You just need to do your thing and shoot the best score you can and hopefully it will be somewhere near good enough."
Lowry's round included four birdies and a bogey, while he drained a 20-foot putt on the sixth hole to save par.
Canadian Adam Svensson is one stroke back from the quartet at five-under, before a three-shot gap to the next in the field.
Pre-tournament favourites Brooks Koepka and Louis Oosthuizen are both well back, at one-over and two-over overall respectively.